The protein of human serum, tentatively designated ß1C-globulin, was shown to possess serological activity and to be related to the complement system. Another serum protein (ß1A-globulin) was identified as the inactivated form of ß1C-globulin.

Incubation of fresh serum with various immune precipitates or with soluble γ-globulin aggregates at 37°C. resulted in the removal of ß1C-globulin. Treatment of fresh serum with zymosan at 17 and 37°C. had a similar effect. In both instances ß1C-globulin was removed from serum, apparently by conversion to ß1A-globulin. However, isolated ß1C-globulin did not react with immune precipitates or zymosan, nor did ß1C-globulin of serum previously heated at 56°C.

Highly purified ß1C-globulin was tested for complement component activity by means of the usual reagents. All of the preparations examined were found to reconstitute the hemolytic activity of guinea pig R3. However, they failed to reconstitute R3 obtained from human serum. Isolated ß1A-globulin was found to be inactive in all systems.

When isolated ß1C-globulin in either phosphate or in borate buffer was stored at 37°C., the activity detected by means of guinea pig R3 declined within 6 days to 20 to 30 per cent of its original value. As the activity decreased, ß1C-globulin was gradually converted to ß1A-globulin.

Addition of ß1C-globulin to a limited complement system (human C') caused an increase of both initial velocity and final degree of hemolysis.

Although ß1C-globulin did not cause lysis of EAC'1, 4, 2, it fully prevented the otherwise rapid decay of EAC'1, 4, 2 at 37°C., and so presumably interacted with this complex.

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