Immunohistochemistry and image analysis were used to quantify alterations in the Kupffer cell and 'activated' perisinusoidal cell populations in the different stages of primary biliary cirrhosis. Anti-CD68 macrophage antibodies were used to detect Kupffer cells, and anti-alpha-smooth muscle actin (alpha-SMA), PR 2D3 and anti-desmin antibodies to detect perisinusoidal cells. Liver biopsy material was available from 26 patients with primary biliary cirrhosis and 23 patients with histologically normal liver. Increased Kupffer cell numbers were observed in periportal/periseptal zones of stage 3 primary biliary cirrhosis (n = 9), and in random parenchymal areas of stage 3 and stage 4 cases. Significantly increased 'activated' perisinusoidal cell numbers were seen only in periportal/periseptal zones of stage 3 and stage 4 primary biliary cirrhosis. Neither Kupffer cell nor perisinusoidal cell numbers altered significantly in stage 1 and 2 primary biliary cirrhosis (n = 6). PR 2D3 positivity and increased alpha-SMA immunoreactivity by perisinusoidal cells in primary biliary cirrhosis support myofibroblastic differentiation of these cells. Human perisinusoidal cells, unlike their rodent counterparts, did not express desmin in primary biliary cirrhosis or control liver. Kupffer cells and 'activated' perisinusoidal cell accumulation in periportal/periseptal zones of the precirrhotic and cirrhotic primary biliary cirrhosis liver support the concept of Kupffer cell-mediated stimulation of perisinusoidal cells. Furthermore, these findings indicate that Kupffer cell-perisinusoidal interactions play an important role in the development of liver fibrosis and cirrhosis in primary biliary cirrhosis.