Studies in experimental human lung cancer models have suggested that interferon may enhance significantly the response to some cytotoxic drugs. We have performed a phase II study of cisplatin (100 mg/m2 q.21 or 28 days) and alpha-2 interferon (3 or 5 MU three times weekly) in 68 patients with advanced non-small cell lung cancer and good performance status. As toxicity was acceptable, the dose of interferon and schedule of cisplatin were increased at the midpoint of the study. 46% (11/24) of patients with squamous carcinoma responded and an overall partial response rate of 30% was attained in 60 evaluable patients. There was no potentiation of haematological, renal or neurological toxicity but nausea and vomiting were severe. These results suggest that the combination has activity in this usually refractory disease.