ABSTRACT:Heterologous anti‐HCG and anti‐β‐HCG sera failed to effect complement‐dependent lysis of purified51chromium‐labeled syncytiotrophoblast in vitro. The progesterone synthetic activity of syncytiotrophoblast was also preserved in the presence of the anti‐HCG sera. Binding of the anti‐HCG antibodies to the syncytiotrophoblast was demonstrated in an immunofluorescence test. The potential of the anti‐HCG sera to participate in complement‐dependent lysis of other HCG‐bearing cell types was revealed using BeWo cells as the targets; 53%‐68% of the latter were killed by exposure to anti‐HCG sera and complement.The possible mechanism(s) whereby syncytiotrophoblast resists damage by the anti