Fat storing cells (FSCs) in the liver represent the main site of vitamin A deposition in the body. These cells are considered to play an important role during scar formation and fibrogenesis in the liver. The putative descent of FSCs from the fibroblastic or from the myofibroblastic system have not been determined yet by morphological or immunohistochemical studies. To further define the origin of these liver cells, we analysed the pattern of expression of three structural proteins: vimentin, desmin and the α-smooth muscle (SM)-actin isoform in FSCs of the rat liver, in smooth muscle cells (SMCs) from the aorta and in rat skin fibroblasts. FSCs were studied by immunohistochemical methods immediately after isolation, at days 3 and 7 after plating. FSC-geneexpression was also analysed by Northern blot analysis of total RNA extracted from cells in culture at days 3 and 7 after isolation. Arterial SMCs and skin fibroblasts were studied in a similar way. For comparison, isolated rat hepatocytes and Küpffer cells (Kc) were studied. Of freshly isolated FSCs, 100% were vimentin-positive, 50% were desmin-positive, but all were α-SM-actin negative. Three days after isolation, FSCs were clearly positive for vimentin and desmin and weakly α-SM-actinpositive, as demonstrated by indirect immunofluorescence as well as by the immunoperoxidase technique. Desmin, α-SM-actin and vimentin staining was further increased at day 7 after isolation, and α-actin specific transcripts in FSC-RNA were clearly detectable at day 7 after isolation. Passaged arterial SMCs were vimentin-and α-SM-actin-positive, but desmin-negative and fibroblasts were only vimentin-positive. α-SM-actin isoform gene expression was strongly increased in the rat liver after acute or chronic damage induced by treatment with carbon tetrachloride. Positive staining was detected in cells which were also desmin-positive. As FSCs are vimentin-, desmin-and α-SM-actin-positive, we suggest that they are smooth muscle-related myo-fibroblasts with special functions.