We have studied the intensity of staining of helper/inducer T-cells in lymph node and tonsillar tissue using two commercially available monoclonal antibodies (OKT4 and Leu3a) with the indirect immunoperoxidase method. Paracortical and mantle zone helper/inducer T-cells were easily visualized by both monoclonal antibodies, but T-cells in the follicular center, though stained by Leu3a, were hardly demonstrable by OKT4. Excellent staining was obtained in the indirect immunoperoxidase procedure by incubating the sections with a 1:1 mixture of the two monoclonal antibodies which gave bright staining of individual cells throughout the lymphoid tissue. Dilution of the primary antibodies by 1:200 did not affect the results. It is concluded that the simultaneous application of OKT4 and Leu3a as primary antibodies in the indirect immunoperoxidase procedure is the method of choice for the in situ demonstration of helper/inducer T-cells.