An immunoradiometric assay has been used to determine the rate of loss of host erythrocyte antigen from the tegumental surface of adult Schistosoma mansoni. Host antigens were lost from the parasite surface with a half-time of up to 45 h during chase incubation in vitro, and with a half-time of about 5 days during residence in vivo in naive recipient hamsters. Since the host antigens were associated intimately with the outer bilayer of the tegument surface, it is suggested that the rate of surface turnover may similarly be very low. The implications of these findings for immune evasion by the adult parasite are discussed.