Little is known about the mechanisms that control transformations during the life cycle of Schistosoma mansoni. To enable isolation of DNA sequences encoding developmentally regulated antigens a cDNA expression library in the vector Xgtll amp3 was constructed from adult mRNA and immunologically screened with sera from infected individuals. We report here on the properties of three recombinant clones that derive from developmentally regulated genes. Clone 10-3 encoded a .3-galactosidase fusion protein present in high abundance in infected Escherichia coli. Clones 7-2 and 8-2 also produced immunologically recognized proteins; however, the peptides did not appear to be fi-galactosidase fusion proteins.