The phenotype of a lymphoid orbital tumor is described, using monoclonal antibodies of the OKT series. The majority of tumor cells lacked surface immunoglobulins and bound OKT3, hence they arose from the T lineage. These cells belonged to the mature helper T cell subpopulation since a great proportion was recognized by OKT4 antibody. In contrast, a decrease in the peripheral blood OKT4+ pool was observed. The results demonstrate the importance of using a panel of markers, such as monoclonal antibodies, for the identification of cell derivation of lymphomas.