Antibody and lectin binding characteristics of Schistosoma mansoni schistosomula maturing in vivo and in vitro were quantitatively assessed and compared in order to investigate the basis of the reduced surface antigenicity of host derived larval schistosomes. Quantitative indirect immunofluorescence assays showed that schistosomula recovered from mice at 24 h and 5-10 days post infection bound low or insignificant amounts of a variety of anti-schistosome antibodies including those from chronically infected and radiation attenuated cercariae-vaccinated mice, a vaccinated rabbit and rabbits hyper-immunized with non-living larval and adult schistosome antigen preparations. In contrast, parasites maturing in vitro continued to bind highly significant levels of each of these antibody preparations until at least 10 days post transformation. To investigate the basis of the decreased surface antigenicity of parasites maturing in vivo, 6-day-cultured parasites were injected intravenously into mice and recovered from the lungs at various times thereafter and examined for their ability to bind both anti-parasite and anti-host antibodies. After 30 min in vivo, cultured schistosomula exhibited a significantly decreased capacity to bind anti-parasite antibodies and concanavalin A (Con A), and by 16 h had lost their binding sites for fucose binding protein (FBP) as well. That this reduction in antigenicity was due to shedding of surface antigens was suggested by the observation that the reduced ability of these parasites to bind anti-parasite antibodies coincided closely with the loss of 125I-labelled surface proteins. Furthermore unlike 6 day schistosomula which had developed wholly in vivo, 6-day-cultured parasites recovered after 30 min in vivo failed to bind anti-host antibodies suggesting that in these organisms parasite antigens were not masked by host molecules. These data argue that surface antigen shedding may explain the reduced surface antigenicity of schistosomula developing in vivo. While this surface modulation apparently occurs independently of host antigen uptake, it is dependent upon an as yet unidentified host factor.