Monoclonal antibodies, either alone or conjugated to toxins, hold promise as important therapeutic agents. However, the immune response to these foreign protein agents may markedly limit their therapeutic utility in vivo. We have administered both an interleukin-2 receptor-specific monoclonal antibody (anti-IL-2R) and a CD2-specific monoclonal antibody linked to the ribosome-inactivating protein gelonin to macaque monkeys. The monkeys developed high-titer antibody responses to mouse Ig and, when immunotoxin was administered, to the toxin gelonin. Their antimouse Ig antibody responses were broadly reactive with mouse Ig of differing idiotypes and isotypes. Furthermore, sera from these monkeys blocked the in vitro cytotoxic effect of anti-IL-2R or immunotoxin. This blocking was mediated by both the antimouse Ig and the antigelonin antibodies. Serum from a monkey infused with one CD2-specific monoclonal antibody blocked the in vitro cytotoxicity of two other isotypically different CD2-specific monoclonal antibody conjugates. In addition, this serum blocked the in vitro cytotoxicity of a gelonin-monoclonal antibody conjugate of an unrelated specificity. These data indicate that the immune response to some monoclonal antibodies and toxins might preclude the later use of this class of substances in an individual. Therefore, strategies for the parental therapeutic use of monoclonal antibodies and immunotoxins must take into consideration the possible limiting effects of the humoral immune response to these agents.