The epitope recognized by monoclonal antibody directed against the HIV-1 recombinant gp160 protein was precisely delineated by using a number of peptides comprising amino acid positions 302-330 of the protein. Two different enzymes, glucose oxidase and horseradish peroxidase, were then coupled to distinct antibody molecules and the efficacy of the immunoenzymes in killing yeast cells which express the recognized peptide was evaluated by flow cytometry analysis. The antibody-glucose oxidase conjugate alone was cytotoxic only at large doses (over 35 micrograms/ml) while in the presence of the antibody-horseradish peroxidase conjugate, killing was observed at nine times lower concentrations (4 micrograms/ml). The procedure described here may provide a new immunotherapy tool for microbial infection.