1. Moving-boundary electrophoresis of the mucin from the Cowper's gland of the boar revealed a sharp single peak at pH values from 1.1 to 9.0 and an isoelectric point of 1.1. 2. Neuraminidase treatment of the mucin, which removed at least 96% of the sialic acid groups, decreased the electrophoretic mobility at pH4 from -7.4x10(-5) (for the mucin) to -0.64x10(-5)cm(2)V(-1)s(-1). 3. Ultracentrifugal sedimentation values of s(20,w) showed a marked dependence on concentration. A hyperfine peak, similar to that given by ovine submaxillary secretion, persisted throughout the run at higher concentrations. Ultracentrifugal studies further showed a very low value for the diffusion coefficient (D(20,w) -1.57x10(-8)cm(2)/s). 4. Calculation of the approximate molecular weight from comparable s(20,w) and D(20,w) values gave a provisional value of 6.5x10(6). 5. Two proteins present in the boar vesicular secretion known as protein A and protein H (the haemagglutinating protein) were shown to promote the swelling of the mucin to form the characteristic rigid elastic gel of boar semen. It is suggested that protein A molecules particularly (mol.wt. 2.8x10(4)) cross-link with the long molecules of the mucin to form the seminal gel.

Forty-five samples of seminal plasma obtained from eight boars and seventeen samples of vesicular secretion collected from individual boars had mean zinc contents of 22\m=.\7and 137\m=.\4 \g=m\g/ml, respectively. It was concluded that most of the zinc present in the seminal plasma of the boar is derived from the vesicular secretion.The mean concentrations of the organic constituents studied were: Seminal plasma: total nitrogen 4\m=.\42mg/ml (non-dialysable 88\m=.\7%), citrate 1 \m=.\28 mg/ml. Vesicular secretion: total nitrogen 15\m=.\1 mg/ml (non-dialysable 93\m=.\9%), citrate 6\m=.\07mg/ml, fructose 0\m=.\557mg/ ml, galactose 0\m=.\092 mg/ml. In both seminal plasma and vesicular secretion, the concentration of zinc was positively correlated with that of total and non-dialysable nitrogen and citrate. The correlation with nitrogenous substances was particularly strong in the seminal plasma of individual animals and there was evidence from the partial correlation coefficients that these associations were independent of the zinc-citrate relationship. However, fructose and galactose which do not have an appreciable affinity for zinc under physiological conditions also showed significant positive correlations with zinc (in vesicular secretion) and most of the organic constituents were interrelated in this way, indicating that in such data the influence of chemical combination may be masked by that of other factors which regulate the composition of these fluids.

Further studies, using gel filtration on Sephadex G-200 columns, have confirmed and extended the similarities between the major proteins of boar seminal plasma and the vesicular secretion. The haemagglutinin in the vesicular secretion has been shown, by red cell agglutination, to occur as a distinct peak masked by the greater protein content of Fraction A. Completely fresh vesicular secretion kept at 37\s=deg\C and studied at this temperature within 30 min of slaughter, presents an identical gel filtration pattern.

The protein patterns of boar seminal plasma, vesicular secretion and epididymal plasma and of their fractions obtained by gel filtration and dialysis were further studied by gel disc electrophoresis and isoelectric focusing on polyacrylamide. The majority of the seminal plasma proteins originate in the seminal vesicles and the quantitative contribution of the epididymides to the protein pattern is small. Proteins A, B and H were found to be mixtures of large numbers of proteins which were identified and characterized.

The haemagglutinating protein (or proteins) have been separated from boar seminal plasma largely by gel filtration methods. Electrophoretic studies have shown that the haemagglutinating protein material (now designated Protein H) is positively charged and that it possesses an isoelectric point (i.e.p.) of about 9\m=.\4.Earlier electrophoretic observations on mixtures of boar seminal plasma proteins have been verified with the greatly purified Protein A (i.e.p. 8\m=.\8) and Protein B (i.e.p. 4\m=.\6). It has been possible to explain some hitherto puzzling properties of the boar seminal plasma caused by isolation of various combinations of Protein A, Protein B and Protein H. * The first paper in this series, (Boursnell, Cole & Briggs, 1968b), was unnumbered. f Postal address: Animal Research Station, 307 Huntingdon Road, Cambridge CB3 OJQ..