We gave attention to the following as specific immunological tolerance factors in pregnancy: a) the serum levels of circulated immune complex (IC), b) serum complement levels (C1q, C3, C4, C1-Inhibitor). The results were as follows: In spontaneous abortions measured by C1q binding test (Zubler et al.) IC levels did not differ significantly from those in normal pregnancies in the first trimester, but in both cases, IC levels were slightly higher than the levels in nonpregnant women. C1q and C1-Inhibitor levels in normal pregnancies were significantly lower than in nonpregnant women, and both levels in spontaneous abortions were significantly higher than in normal pregnancies. In cases of artificial and spontaneous abortions, increased IC levels were rapidly lowered to the normal range within 7 days after treatment and lowered C1q and C1-Inhibitor levels also returned to the normal range within 7 days. But these trends were more remarkable in artificial abortions. However, C3 and C4 levels did not change after treatment in either case. A negative correlation between complement levels (C1q, C3, C4) and IC levels in the third trimester was demonstrated in our study.