ABSTRACT: Antibodies were raised in rabbits to purified human term placental villous plasma membrane. These were cytotoxic to human peripheral blood lymphocytes and manifested cross-reactivity to kidney and liver. After absorption with these tissues, reactivity was retained with placental villous plasma membrane. The placental-specific antiserum was fully reactive with BeWo choriocarcinoma cells, but was devoid of reaction with a human myeloma tumor cell line (GM2132). The antibodies were cytotoxic to BeWo in the presence of complement, as determined by trypan blue uptake, 51Cr release, and inhibition of uptake of 3H-thymidine.