A phase II study was performed to evaluate the clinical and immunological effects of a regimen of cisplatin (DDP) and etoposide (VP-16) combined with thymosin-alpha 1 (TA1) and low-dose interferon-alpha 2a (IFN) in the treatment of patients with advanced non-small cell lung cancer (NSCLC). Chemoimmunotherapy cycles were repeated every 3 weeks. There were 24 responses (two complete, 22 partial) among 56 assessable patients. Median survival was 12.6 months. Overall, treatment was well tolerated. Natural killer cell activity and lymphocyte subtypes were depressed by chemotherapy, but this effect was less prominent in patients receiving TA1 and IFN in comparison with a concomitant group of patients treated with DDP and VP-16 only. The combination of DDP and VP-16 and TA1 and IFN is effective in advanced NSCLC with acceptable toxicity. However, the results of this study need to be confirmed in a randomised trial.