Human trophoblast membrane antigens recognised by monoclonal antibodies (H310, H315, H316 and H317) have been identified using combinations of radioimmunoprecipitation, SDS-PAGE, electroblotting, chromatographic and ELISA-type techniques. H317 is known to identify heat-stable placental-type alkaline phosphatase and accordingly was shown to react with a protein of subunit Mr of 68 000. H310 and H316 both recognise an antigen with a subunit Mr of 34 000 under reducing conditions. In non-reducing conditions, the H310/316 antigen gave oligomers of a component of Mr 62 000. It is unknown whether this 62 000 dalton component is a dimer of the 34 000 dalton protein with either itself or a second protein chain of presumed Mr around 28 000. H315 recognises an antigen with subunit Mr of 36 000; in non-reducing conditions this component readily associates to oligomeric structures. The epitope recognised by H315 may be sensitive to SDS. The two proteins recognised by H310/316 and H315 have been termed the p34 and p36 trophoblast membrane proteins, respectively.