Ninety patients with histologically documented chronic non-A, non-B hepatitis were randomly allocated to receive SC injections of placebo or of 1 or 3 MU of recombinant interferon alfa-2b three times weekly for 24 weeks. Complete normalization of alanine aminotransferase levels occurred posttreatment in 43.3% of patients receiving 3 MU, in 20% of those receiving 1 MU, and in 6.7% of untreated patients (P less than 0.0005 vs. those treated with 3 MU). Alanine aminotransferase normalization was sustained for 6 months after therapy in 13.3% of the patients treated with 3 MU and in 3.3% of those given 1 MU or placebo. The decline of alanine aminotransferase levels following interferon therapy showed independent, positive correlations with female sex (P less than 0.03) and younger age (P less than 0.05). The Knodell's fibrosis score was strongly positively correlated with age (P less than 0.0001). It is concluded that 3 MU of interferon is a more effective dose than 1 MU for controlling disease activity in non-A, non-B chronic hepatitis patients. Women and younger and noncirrhotic patients are more likely to respond.