Factor B of the alternate pathway participates in the activation of complement component C3. Factor B and fragments thereof have been isolated from serum and partially characterized. Highly purified factor B is homogeneous by agarose electrophoresis but electrophoretic techniques with higher resolution and isoelectric focusing reveal two types of charge heterogeneity for factor B. The heterogeneity is thus partly dependent upon a genetic polymorphism but in addition a nongenetical microheterogeneity is observed. The sedimentation constant for factor B is 5.9 S and the Stokes' molecular radius is 40 A. The molecular weight of factor B was assessed by several techniques and a value of about 90 000 was established with all methods. These data demonstrated that factor B consists of a single polypeptide chain. The "1-terminal amino acid residue was shown to be proline. Factor B contains about 7.3% carbohydrate. Two fragments of factor B were also encountered in serum. The quantitatively dominating component had a molecular weight of 63 000, was basic in character, and did not exhibit any high degree of charge heterogeneity. Occasionally a basic fragment with the molecular weight 47 000 was also obtained. Immunological analyses revealed that both fragments displayed less antigenic sites than A key step in the complement reaction sequence is the activation of C3 (see Muller-Eberhard, 1975). This can occur principally in two ways. The classical activation mechanism involves factors C1, C4, and C2 but an alternate pathway is also existent. Factors A, B, and D can accordingly participate in the formation of a C3 convertase (for a review, see MullerEberhard, 1971). Factor B (C3 proactivator) was first isolated by Gotze and Muller-Eberhard (1 971). They estimated the molecular weight of factor B to 80 000 and showed that on activation the protein lost an acidic fragment with an apparent molecular weight of about 20 000 leaving the C3-cleaving activity associated with a basic polypeptide with the molecular weight 60 000. Haupt and Heide (1965) and Boenish and Alper (1970a) isolated independently a serum glycoprotein with @l to y mobility on electrophoresis. The latter authors noted that the isolated protein probably represented a fragment of a larger glycoprotein, which subsequently was successfully isolated (Boenish and Alper, 1970b). This protein, termed glycine-rich 0-glycoprotein, was shown to display two major electrophoretic forms, S and F, which strongly indicated a genetically controlled polymorphism in the glycine-rich P-glycoprotein (Alper et al., 1972). In addition several minor, nongenetically deter-

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Papain-solubilized murine transplantation antigens have been isolated from spleen cells. The isolated H-2 antigens, containing two types of subunits, were homogeneous in size but displayed charge heterogeneity. On isoelectric focusing under denaturing conditions the H-2 antigens resolved into several components. The most basic material (pI = 7.1) comprised exclusively the smaller subunit (fo-microglobulin) and appeared homogeneous in charge. The more acidic material representing the larger, alloantigenic subunit gave rise to several peaks. Separate analyses of H-2K and H-2D antigens gave indistinguishable profiles suggesting that the charge