Four-week-old Schistosoma mansoni collected from experimentally infected albino rats were shown by a fluorescent antibody technique to possess several serum proteins of host origin, including IgM, IgG, complement and red blood cell components. The capacity of these rat-derived worms to acquire and shed host proteins was also demonstrated in vitro with fluoresceinated antibody-labelled worms in the presence of xenogeneic molecules. The implications of these results are discussed in relation to the mechanisms of parasite evasion and "self-cure" phenomenon.