The proportion of lymphocytes from patients with non-Hodgkin lymphoma (NHL) expressing transferrin receptors in biopsy material from lymph-nodes assayed by binding of the monoclonal antibody OKT9 correlated significantly with the histological class of the tumour. High-grade lymphomas contained 22.5% (range 3-57%) OKT9-positive lymphocytes, and low-grade lymphomas contained 2.5% (range less than 1-22). Since histological class of lymphoma is an important factor in survival, it was expected and confirmed that OKT9 levels were related to survival in these patients. Transferrin receptor is expressed by growing cells. The correlates observed in this series of NHL suggests a relation between transferrin receptor and either the growth fraction, or factors affecting the growth fraction, of the tumour.