Some biological and physico-chemical properties were tested in 50 lots of transfer factor (TF). Each lot contained the extracts of a pool of 20 buffy coats, each originating from a 400 ml blood donation of a healthy individual. The optical densities at 260 and 280 nm were within a narrow range in all TF lots. The ratio 260/280 was 2.56-3.02. One unit TF (defined as the extract of two buffy coats (corresponding to 1-2 x 10^9 leucocytes) contained 36.58 ± 3.77 mg of peptides as measured by the Lowry method and 123.6 ± 4.5 mg ribose as estimated by the orcinol method. Cytotoxicity of TF lots was tested by measurement of chromium release of PHA-stimulated lymphocytes. Cytotoxicity was above 10% in 20 of 27 TF lots tested, most of which were produced by an Amicon ultrafiltration cell. Pyrogenicity was present in 8 of 42 TF lots produced by Amicon filters, but only in 1 of 8 lots produced by Sartorius filters. Sephadex G-10 chromatograms of TF gave characteristic patterns yielding 8 peaks. Sephadex G-25 chromatograms reproducibly showed 5 peaks. The distribution of osmotic activity, peptides and ribose in respect of the chromatographic fractions is given.