A modified avidin-biotin immunoperoxidase technique has been used to enumerate B lymphocytes, T inducer/helper (TH) and T cytotoxic/suppressor (TS) cells in the peripheral blood of normal and immunosuppressed rhesus monkeys (Macaca mulatta) prior to and following adherent-cell depletion. The levels of each of the B, TH and TS cells detected in the normal monkeys using monoclonal antibodies which recognized specific surface antigens on human lymphocytes were comparable to the levels reported in human peripheral blood using direct immunofluorescence, immunoperoxidase or flow cytometry techniques. Adherent cell depletion did not result in a significant loss of any of the lymphocyte subpopulations examined. The technique is reproducible and sensitive in detecting differences between normal and immunosuppressed animals, and would prove to be useful in studies pertaining to chemical and drug immunomodulation.