In 14 of 16 patients with chronic active hepatitis (C.A.H.) who did not have HLA antigens B8 and/or B12 an external triggering factor (drug or virus) could be demonstrated at onset of symptoms. In contrast external factors were involved in only 11 of 25 cases of C.A.H. in patients with HLA-B8 and/or B12. In the latter group antinuclear antibodies were less common in cases possible triggered by external agents compared with cases in which no such factor was demonstrated. The results suggest that there are at least two pathogenetically different types of C.A.H.---one genetically determined type in which no external factor is involved and in which autoimmune phenomena are common, and another type triggered by environmental agents and not involving predisposing genetic factors.