All N:NIH(S)-B mice examined have multiple foci of hepatic necrosis and inflammation. The cause of this hepatitis is unknown. This strain was developed at the NIH Animal Genetic Resource for use as foster mothers in the barrier facility. All mice appear clinically normal, but have significant abnormalities in their livers. Numerous studies are in progress to determine the etiology and to explore the pathogenesis of hepatitis in this strain. Vitamin A is being examined as a possible etiology or co-factor. Mouse strains closely related to the N:NIH(S)-B are being examined for hepatic abnormalities. Hepatitis in this strain may serve as a useful model to study the pathogenesis of hepatitis in animals and humans.