The effects of hymostimulin on chemotherapy-induced toxicity and long-term survival were studied in 26 evaluable patients with small cell lung cancer. Patients were randomly treated with six cycles of two alternating regimens (cyclophosphamide, 4'-epidoxorubicin, and etoposide; cisplatin and etoposide), with (n = 15) or without (n = 11) thymostimulin (1 mg/Kg i.m., days 7-14 of every cycle). Only complete responders received maintenance treatment, consisting of thymostimulin administered 1 mg/Kg i.m., twice weekly, until tumor relapse. Myelosuppression, fever and documented infectious episodes were significantly less severe in thymostimulin-treated patients, allowing the administration of significantly higher drug doses at shorter intervals between chemotherapy cycles; a significant improvement in complete response rate and survival were also observed. Results suggest that the addition of thymostimulin to standard chemotherapeutic regimens might be of benefit, in view of its favourable effects on toxicity and long-term survival.