Concentrations of IgG, IgA, IgM and IgD were measured against WHO Reference Preparation 67/97 and British Standard 67/37 in sera from 105 patients with viral hepatitis and 110 "current" controls. To allow comparison at different ages and between sexes, constants previously determined on sera from a large "laboratory" group of controls were used to calculate "scores" for all Ig values. The patients' IgM scores were remarkably higher than those of controls and were bimodally distributed. About 68% of the patients had IgM scores more than 2 SD above the expected mean for laboratory controls. The distribution of IgG and IgA scores appeared unimodal, but schifted to the right of controls. The mean values of IgG and IgA scores were significantly greater in patients than in controls, and in female patients compared with males. Moreover, 16% (12 of 73) of the male and 47% (15 of 32) of the female patients had Ig G scores more than 2 SD above the expected mean. With regard to IgD, no significant difference between patients and controls was observed.