In order to detect the difference in serum gamma-interferon levels between patients with nasopharyngeal carcinoma (NPC) and normal controls, the reversed passive hemagglutination (RPHA) method was used. Serum gamma-interferon level was determined in 93 patients with NPC, of these, 47 were newly diagnosed, untreated patients. Of the treated patients, 15 had received radiotherapy but were proved later to have distant metastasis. Thirty-one patients had received radiotherapy and survived over 5 years without local recurrence or distant metastasis. Another 47 healthy subjects, age and sex matched with fresh NPC cases, were used as control group. The results revealed that the concentration of gamma-interferon in the serum of neither males nor females showed significant difference (p greater than 0.1). Among the NPC fresh cases, the concentration of gamma-interferon also showed no significant difference in patient serum in either early stages (stages I and II) or later stages (stages III-V) (p greater than 0.1). The serum gamma-interferon level was significantly increased in fresh NPC patients compared with normal controls (p less than 0.05). Concerning the irradiated patients, serum gamma-interferon level was not increased in patients who survived over five years, without relapse, compared with normal controls (p greater than 0.1). However, the serum gamma-interferon level in patients with distant metastasis was higher than in normal controls (p less than 0.05). Also, the serum gamma-interferon level was significantly increased in distant metastatic patients compared with patients without relapse (p less than 0.05).