Monoclonal antibodies that bind to surface membranes of developing schistosomula and/or cercarial tails were generated from mice immunized with living schistosome eggs or soluble egg antigen. These monoclonal antibodies detected at least three different surface epitopes. One surface antigen detected by anti-egg monoclonal antibody EG1C4B1 (E.1) persisted on the surface of developing schistosomula for 96 h posttransformation . The same or a cross-reactive antigen was also detected on the surfaces of S. japonicum and S. haematobium schistosomula and cercarial tails. Monoclonal antibody E.1 killed schistosomula in vitro as well or better than infected mouse sera and transferred immunity to naive mice when administered in vivo. The monoclonal antibody reduced the number of lung worms recoverable on day 4 postchallenge by up to 85% and reduced the adult worm burden up to 41% as compared with controls. The data also show that the molecular weights of the egg antigens detected by monoclonal antibody E.1 were different from those detected on schistosomula.