Seventeen patients with small cell lung cancer were entered into a dose ranging phase I-II study using rhGM-CSF (Glaxo). In the phase I study patients received 50, 150, 300 or 500 micrograms/m2 GM-CSF for 10 days by daily subcutaneous injection. Full blood counts were performed thrice weekly. After 4 days off all therapy patients then received chemotherapy with doxorubicin 50 mg/m2 i.v. bolus, day 1, ifosfamide 5 g/m2 with mesna 5 g/m2 over 24 h by continuous infusion followed by mesna 3 g/m2, and etoposide 120 mg/m2 i.v. on days 1-3. A total of six courses of chemotherapy were given. In the phase II study patients received the same dose of GM-CSF as in the phase I. GM-CSF was given 24 h after the last dose of chemotherapy for 14 days. Full blood counts were checked thrice weekly and the incidence of infections noted. Patients were randomised to receive GM-CSF with either odd or even courses of chemotherapy. The leucocyte count rose from a mean of 8.7 to 21.6 x 10(9)/l at the 50 micrograms/m2 GM-CSF dosage and from 11.4 to 39.4 x 10(9)/l at the 500 micrograms/m2 dosage during the phase I study. Phase I toxicity was: bone pain in 65% of patients, rash in 47%, fever in 24%, lethargy in 12% and diarrhoea in 12%. In the phase II study the duration of neutropenia was less during the chemotherapy courses with GM-CSF (p = 0.04) but the number of infections was similar.(ABSTRACT TRUNCATED AT 250 WORDS)