The marked and quick response to betaleukin, a preparation of recombinant human interleukin-1 beta, was demonstrated in 77 cases of extensive solid neoplasms and non-Hodgkin's disease, suffering combination chemotherapy-induced toxic leukopenia. Leukopoiesis was effectively stimulated by intravenous administration, of 10-20 ng/kg weight, dropwise for 5 days. Due to adjuvant betaleukin in identical dosage, chemotherapy was continued, in moderate leukopenia, without causing leukocyte count to drop. The untoward side-effects (chiefly. hyperthermia including fever) were reversible and did not hamper treatment.