Immunoglobulins, mostly of the IgG class, were detected in eluates of the placenta of 75% of 50 healthy women in their first or second pregnancy, 92% of 30 women with more than two pregnancies, and 87% of 23 pre-eclamptic patients. The immunoglobulins were assayed for complement-dependent cytotoxicity on human and monkey cell-lines, as well as on the same cells chronically infected with either Mason-Pfizer Virus (M-P V) or Baboon Endogenous Virus (BeV). The frequency of cytotoxic reactions was very low, except with immunoglobulins from the pre-eclamptic placentae, where one third of the samples lysed virus-infected cells with occasional killing of virus-free cells. All placental immunoglobulins which were not cytotoxic were then assayed for blocking activity by testing whether they could compete with the action of anticellular sera of virus-free cells, or with the toxic effect of antiviral sera on virus producing cells. 64% of the immunoglobulins from normal placentae competed with antiviral antibodies while only 17% blocked the action of anticellular sera. The frequency of blocking immunoglobulins was no greater in eluates from pre-eclamptic placentae. The data indicate that the placenta possesses retrovirus antigen sites which bind blocking antibodies in normal pregnancy and complement-dependent cytotoxic antibodies in pre-eclampsia.