Interleukin-6 (IL-6) has in vitro demonstrated growth regulatory effects on tumor cells from patients with chronic lymphocytic leukemia (CLL) and lymphoma. The proliferation rate of these cells is usually very low and this is thought to be one of the reasons for the lack of a curative potential of cytostatic chemotherapy in CLL and low grade NHL. Recombinant human (rh) IL-6 might increase the in vivo proliferation rate leading to a higher sensitivity for chemotherapy. We tested this hypothesis by administering rhIL-6 to 9 CLL patients and 3 NHL patients in doses of 2.5 micrograms/kg, 5 micrograms/kg and 10 micrograms/kg s.c. daily for 5 days followed by CHOP chemotherapy on the last day of rhIL-6 injection. Six patients had two treatment cycles. The proportion of cells in S-phase was determined by the bromodeoxyuridine labeling index (LI). Three patients achieved a partial remission, one patient had progressive disease and the remaining patients demonstrated no change. Two patients, who received 10 micrograms/kg/day rhIL-6, demonstrated a significant increase in LI, one of these was first observed in the second treatment cycle. A significant decrease was seen in two patients receiving 2.5 micrograms/kg and 5 micrograms/kg respectively. Immunophenotypic assessment demonstrated that rhIL-6 increased the expression of CD20 in all CLL patients with a reversal after cessation of rhIL-6. We conclude that rhIL-6, in the dosage and schedule used in this study, did not increase the proportion of the cells in S-phase and that the growth stimulatory effects of rhIL-6 in CLL in vivo probably are insignificant. However, the role of rhIL-6 in CLL as inducer of increased CD20 expression prior to anti-CD20 antibody treatment remains to be determined.