Incubation of fresh serum with various immune precipitates or with soluble -globulin aggregates at 37°C. resulted in the removal of ß_1C-globulin. Treatment of fresh serum with zymosan at 17 and 37°C. had a similar effect. In both instances ß_1C-globulin was removed from serum, apparently by conversion to ß_1A-globulin. However, isolated ß_1C-globulin did not react with immune precipitates or zymosan, nor did ß_1C-globulin of serum previously heated at 56°C.

Highly purified ß_1C-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 ß_1A-globulin was found to be inactive in all systems.

When isolated ß_1C-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, ß_1C-globulin was gradually converted to ß_1A-globulin.

Addition of ß_1C-globulin to a limited complement system (human C') caused an increase of both initial velocity and final degree of hemolysis.

Although ß_1C-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.