15 patients with primary biliary cirrhosis (PBC) and 109 patients with chronic aggressive hepatitis (CAH) have been followed. Features of PBC, namely the generalized pruritus, massive rise in alkaline phosphatase, antimitochondrial antibodies and high levels of IgM-globulins, were present in 7 patients with CAH. This group was treated with immunosuppressive drugs for 1-2 years. Clinical, biochemical, immunological and histological parameters were used to assess the therapeutic effect. The pruritus improved and there was a statistically significant reduction in the IgG-hyperglobulinemia. Some resolution of the piecemeal necroses was seen. However, in judging these changes the sampling error must be taken into account. The unknown agent attacks both the hepatocytes and the epithelial cells of the bile ducts. The immunosuppressive treatment protects the liver cells from further damage while the progressive destruction of the bile ducts remains uninfluenced. The results suggest that the smallest possible dose sufficient to suppress the activity of CAH must be selected.