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Antigenic proteins on the surfaces of different developmental stages of Schistosoma mansoni were radio-iodinated by the Iodogen-catalysed method and identified by immunoprecipitation with a panel of antisera. The sera comprised specific immune serum from mice harbouring a chronic schistosome infection or vaccinated with gamma-irradiated cercariae; serum from rabbits immunized with adult schistosome tegumental outer membranes or a partially purified Mr 32 K glycoprotein from adult worm membranes; and a monoclonal antibody recognizing an Mr 20 K antigen on the surface of schistosomula. The Mr 38-32 K glycoproteins were the major antigens identified in surface-labelled cercariae and their probable association with the glycocalyx is discussed. Schistosomula transformed from cercariae either mechanically or by penetration of host skin in vitro, expressed a similar pattern of surface antigens to that identified for cercariae, but low molecular weight antigens of Mr 20, 17 and 15 K were also detected. The Mr 38-32 K glycoproteins, although present on newly transformed schistosomula, were progressively replaced with time, by a single dominant glycoprotein (Mr 32 K) expressing identical epitopes to those on the Mr 38-32 K complex. Moreover, the data confirm that the Mr 32 K glycoprotein persists on the tegument after in vivo maturation and is conserved, together with Mr 20 and 15 K antigens, through to the adult stage. New antigens (Mr 97 and 25 K) were also detected during in vivo maturation and were present in late-stage schistosomes recovered from infected hosts. In addition, the enzyme alkaline phosphatase is expressed on the surfaces of 3-week-old liver worms as a dominant antigen (Mr 65 K); this feature may be related to nutritional and/or physiological processes in the tegument of this metabolically active stage of development.