Combinations of monoclonal antibodies defining the OKT 4 positive lymphocyte subset in the peripheral blood of intravenous drug abusers suggest that the 4B4 positive helper inducer population is greatly reduced in subjects infected with HTLV III. No differences were seen instead using different antibodies directed against various epitopes of the T4 molecule. These results suggest that the multiple marker assay could be very useful to define immunological status during HTLV III infection in humans.