SummaryA panel of monoclonal antibodies has been shown previously to identify both serologically diverse and serologically conserved epitopes on a major polymorphic surface protein ofP. faliparumschizonts from culture‐adapted isolates. The molecular nature of the antigen recognized by eight of these monoclonal antibodies was studied with three isolates analyzed directly from patients in The Gambia. Malarial (glyco) proteins were labelled by biosynthetic uptake of3H‐glucosamine or3H‐leucine during culture of ring‐stage parasites from infected blood to the late‐trophozoite/early‐schizont stage (26–30 h). Those monoclonal antibodies which reacted positively with an isolate by indirect immunofluorescence also immunoprecipitated a single3H‐leucine or3H‐glucos‐amine labelled antigen of mol. wt ˜200 000 from Triton X‐100 extracts of the same isolate. Monoclonal antibodies which did not react by indirect immunofluorescence failed to immunoprecipitate this antigen. Although each of the three isolates studied in detail was very similar serologically with the panel of monoclonal antibodies specific for this mol. wt ˜200 000 antigen, this protein could be distinguished with each isolate on the basis of its apparent size on SDS‐polyacrylamide gel electrophoresis. The specifically immunoprecipitated antigen had a mol. wt of 204 000, 197 000 or 202 000, depending on the isolate. Size diversity of this malarial glycoprotein was also detected with seven other GambianP. falciparumisolates. We conclude that natural isolates ofP. falciparumexpress a major3H‐glucosamine labelled glycoprotein of mol. wt Mr˜200 000 which exhibits size diversity and expresses antigenically conserved as well as diverse epitopes as defined by the pa