Decay-accelerating factor (DAF) is a 70-kD membrane glycoprotein that prevents complement (C)-mediated hemolysis by blocking the assembly or accelerating the decay of C3 convertase. Purified DAF is known to incorporate into the membrane of DAF-deficient cells, inhibiting lysis. Since Schistosoma mansoni is a blood-dwelling parasite, we investigated whether DAF can be transferred from human erythrocytes to the worm and protect it against C-mediated killing in vitro. We have found that schistosomula (schla) incubated with normal human erythrocytes (N-HuE), but not with DAF-deficient erythrocytes, become resistant to C damage in vitro. Protected parasites acquire a 70-kD surface protein which can be immunoprecipitated by anti-DAF antibodies. The acquired resistance is abrogated by treatment of N-HuE-incubated parasites with anti-DAF antibody. These results indicate that, in vitro, N-HuE DAF can be transferred to schla, and suggest its participation in preventing their C-mediated killing. This could represent an important strategy of parasites to evade the host's immune response in vivo.