Serum complement (C’) activity in recurrent spontaneous aborters and primiparous controls with successful and unsuccessful pregnancies was quantified so as to define the dynamics of C’ activation in early pregnancy loss. C’ hemolytic activity was shown to be stable throughout the first trimester of pregnancy and did not differ from preconception levels in all of the successful pregnancies of recurrent aborters and controls and in the majority of pregnancy losses. However, 30% of recurrent aborters and 20% of controls with a pregnancy loss demonstrated activation of C’ by the alternate pathway as early as the 7th week with a progressive decline in C activity until abortion was clinically completed. Circulating levels of C3 dropped from 1.34 to 0.53 mg/dl, and factor B levels declined from 0.34 to 0.14 mg/dl in these hypocomplementemic women. Pregnancy loss is therefore associated with C’ activation in a subset of both recurrent and nonrecurrent aborters and this occurs largely before loss of fetal viability.