SummaryWe have expressed two cDNA sequences encoding 121 and 230 amino acids of the C‐terminus of theSchistosoma mansoniHsp70 inEscherichia coli.The products were synthesized as polypeptides fused to the RNA polymerase of bacteriophage MS2, and their reactivities were tested in ELISAs, using sera from human and murine infections. Anti‐Hsp70 antibodies were detected in a significant number of individuals suffering from chronic schistosomiasis mansoni, but not in patients with known recent infections. This, together with the finding that antibodies directed atS. manoni‐specific Hsp70 determinants during the course of infection of experimental mice were not detectable until 5–6 weeks postinfection, suggests that the protein may be a useful marker for distinguishing late and early infections. The diagnostic specificity of Hsp70 was evaluated with sera from humans infected with different schistosome species and other parasitic diseases. While some subjects infected with 5.haematobiumproduced antibodies which recognized theS. mansoniHsp70, no such antibodies were generated in 5.japonicuminfected individuals. However, cross‐reactive antibodies were elicited in donors with other parasitic diseases such as filariasis and malaria. The absence of antibodies in early infection and the observed cross‐reactivities led us to conclude that Hsp70 will be of limited value in the diagnosis of schi