ABSTRACT:Further data are presented on the isolation and biochemical analysis of syncytiotrophoblast microvillus plasma membrane preparations from normal full‐term human placentas. No evidence has been obtained that IgG associated with these membranes is maternal antibody to trophoblast antigen. An oncotrophoblast plasma membrane antigen was previously identified by immunofluorescence, and an indirect radioisotopic antibody assay has now been used to confirm the detection of a cell surface antigen common to breast carcinoma cells and normal trophoblast. The relation of trophoblast plasma membrane antigens to possible maternal immunization in pregnancy is discusse