Data are presented on the general and hematological toxicity of a cisplatin (DDP), 5-fluorouracil (5-FU) and alpha interferon (IFN-alpha) association in patients with stage III B or IV non small cell lung cancer (NSCLC). Twenty patients received DDP (100 mg/mq i.v., day 1) and 5-FU (750 mg/mq/day i.v. continuous infusion, days 1 to 4). In ten of these patients IFN-alpha (3 MU s.c., three times weekly, days 1 to 21) was added. General and hematological toxicity was of a similar degree in both groups. Recombinant granulocyte colony stimulating factor (G-CSF; 5 micrograms/kg b.w. s.c. days 7 to 18) induced a sharp increase in peripheral blood GM-CFU level in patients receiving DDP and 5-FU but not in DDP, 5-FU, IFN-alpha treated patients. The results appear to indicate that IFN-alpha modulation of a DDP, 5-FU combination induces an acceptable degree of toxicity.