Murine monoclonal antibodies against staphylococcal alpha-toxin were produced using a well-characterized alpha-toxin fragment preparation as immunizing agent. Three monoclonal antibodies were selected for anti-alpha-toxin activity in an ELISA using alpha-toxin as antigen. The monoclonal antibodies (MAbs) belonged to different immunoglobulin classes/subclasses and showed different abilities to neutralize the hemolytic, cell-membrane-damaging, dermonecrotizing and lethal action of alpha-toxin. One MAb was superior to mouse polyclonal antiserum in all test systems except for hemolysis, whereas another MAb neutralized essentially as the polyclonal serum. The third MAb did not neutralize the hemolytic or dermonecrotic effect but still inhibited the lethal and membrane-damaging effect of alpha-toxin. These results indicate that the three MAbs recognize different epitopes on the toxin molecule and that different biological activities might correspond to these epitopes.