Using a single radial immunodiffusion method, serum immunosuppressive acidic protein (IAP) was determined in 117 patients with primary lung cancer, 34 patients with benign lung disease, and 45 healthy control subjects. The mean value of IAP for patients with lung cancer was significantly higher than that of normal control subjects. There were no significant differences in IAP levels among different histologic types and among different stages of lung cancer. It was concluded that serum IAP level was elevated in patients with lung cancer, especially during the early postoperative period, and serial measurements of serum IAP were of value in monitoring lung cancer patients.