While the central function of the Bb fragment as C3 activating enzyme is well recognized, there are up to now no reports concerning the smaller fragment Ba. To ascertain whether Ba exhibits biological functions the following studies were performed.

Factor B was purified from EDTA-plasma using a seven-step-procedure. The last step was an affinity chromatography at low ionic strength (1 mM MgCl2) on a sepharose 4B column, to which cobra venom factor was coupled. The final product was homogeneous on SDS-PAGE and showed only one precipitation line after immunoelectrophoresis using two sera: goat anti-whole guinea pig serum and goat anti-guinea pig factor B.

Factor B was cleaved by immobilized cobra venom factor and D̄. Fragment Ba was recovered from the filtrate of the reaction mixture while fragment Bb was eluted by 2M NaSCN. The m.w. for B, Bb and Ba as determined by the SDS-PAGE method was 106,000, 65,000 and 53,000, respectively. Reduction did not change the picture, indicating a single chain structure.

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