The proliferation rate of non-Hodgkin's lymphomas (NHL) was estimated by using 3 different methods. In cell suspension we determined the proportion of cells in cycle with the monoclonal antibody (Mab) Ki-67 and also in S-phase after the incorporation of bromo-deoxyuridine (BrdU) utilizing Mab anti-BrdU. In low grade lymphomas 3.5 +/- 1.6% of the cells were in cycle and 1.2 +/- 0.9% in S-phase, the corresponding values for high grade lymphomas were 22.5 +/- 18.7% and 8.9 +/- 7.8% respectively. Frozen sections of NHL were reacted with an antibody to the transferrin receptor (TR) and Ki67 as markers for proliferative activity. A high number of TR positive cells was found in low grade lymphomas of all histological types, whereas Ki67 positivity correlated closely with grading. With a few exceptions, low grade lymphomas contained less than 25% Ki67 positive cells within the tumour cell population. This observation is relevant to treatment strategies for low grade NHL.