Two types of autoimmune hepatitis, with either ASMA-ACTIN (+) or KLM (+) antibodies have been described in pediatric patients. To assess the relationship between HAV virus infection and autoimmune hepatitis with ASMA-ACTIN (+) 18 patients with prolonged (longer than 3 month duration) hepatitis A (IgM HAV +) were studied, 6 boys and 12 girls, whose average age was 8.5 years (range 2-16). Antibodies were assessed on admission to the protocol and 4 to 12 months later. FAN, ASMA-ACTIN and a-KLM antibodies were detected by indirect immunofluorescence (IIF) on rat kidney, stomach and liver sections. Anti actin was confirmed by IIF on rat monolayer cultured fibroblasts. Results: 17 (94.4%) of patients evinced hypergammaglobulinemia (range 2-5 g/l). Both FAN and KLM were negative in 100% of cases. ASMA-ACTIN was positve in 17 patients (94.4%), titers being always higher than 1/40 and up to 1/480. In 8/12 patients (72.7%) anti actin antibodies remained positive, although with lower titers, on the second evaluation. In 3 of 4 patients in whom an hepatic biopsy was obtained this showed chronic active hepatitis. Of the 20 patients with typical acute hepatitis. A 9% had ASMA-ACTIN (-) antibodies, with titers up to 1/80.Conclusions: results suggest an association between ASMA-ACTIN antibodies and prolonged hepatitis A in children. Further investigations should help clarify whether anti actin antibody is associated with HAV or this virus is able to trigger autoimune reactions in the liver.