ABSTRACT— Spontaneous and PWM-driven IgG and IgM synthesis was investigated in the PBMC of 15 patients with HBsAg-negative CAH and six HBsAg-positive patients with CAH. PBMC from patients with HBsAg-positive CAH show an impaired IgG synthesis upon stimulation with PWM but an IgM increase similar to that of control subjects. In contrast, PBMC from HBsAg-negative patients with CAH show a trend to spontaneous increased synthesis of IgG and a decrease or lack of IgG and IgM synthesis upon PWM stimulation. Steroid treatment seems to ameliorate these alterations. No differences were found among the three groups of HBsAg-negative chronic active hepatitis (autoimmune, HBsAg-related and cryptogenic). These results indicate that differences of B-cell functions may exist in the two groups of CAH patients, not only in spontaneous B-cell activation or PWM-induced Ig synthesis but also in the different classes of Ig.