A cell surface antigen has been defined by a monoclonal antibody 1D2, raised following immunisation with lectin-purified syncytiotrophoblast glycoproteins. 1D2 was nonreactive with any one of 8 common trophoblast proteins in immunodot. Analysis of nonreduced western blots of syncytiotrophoblast microvillous plasma membrane (StMPM) protein indicated that mAb 1D2 was reactive with a series of sialylated proteins with molecular weights of 16–22 kilodaltons. Immunoprecipitates of radiolabelled StMPM protein contained molecules that co-migrated with placental alkaline phosphatase in addition to those identified by western blotting. This set of human trophoblast molecules has not been previously identified by monoclonal antibodies; the antigenicity is widely distributed in human tissues.