The monoclonal antibody OKT9 was applied to cryostat sections of 267 non-Hodgkin's lymphomas and related neoplasms. It was found that the transferrin receptor is expressed by a wide variety of B- and T-lineage non-Hodgkin's lymphomas. The OKT9 staining also was loosely correlated with the three morphologic grades of non-Hodgkin's lymphomas identified by the International Working Formulation. In general, higher grade lymphomas more often and more intensely expressed the T9 antigen. However, transferrin receptor expression by certain histologic subtypes of lymphoma did not correlate with their morphologic grade: low-grade follicular lymphomas expressed the T9 antigen more frequently than diffuse low-grade lymphomas; diffuse small cleaved cell lymphomas were stained by OKT9 less often than other histologic subtypes of intermediate-grade lymphomas; and diffuse immunoblastic lymphomas expressed transferrin receptors less often than the other high-grade histologic subtypes of non-Hodgkin's lymphoma. Intermediate lymphocytic lymphomas, not recognized in the International Working Formulation, were infrequently and weakly stained by OKT9 in a manner similar to diffuse low-grade lymphomas. We obtained clinical follow-up data on 43 individuals with chronic lymphocytic leukemia/small lymphocytic lymphoma and 64 individuals with diffuse large cell and immunoblastic lymphoma. Transferrin receptor expression in these two groups did not correlate significantly with survival.