Myofibroblasts are mesenchymal cells that are prominent in liver injury. The origin of myofibroblasts in liver is debated, although morphologic evidence to date has suggested that these cells are derived from lipocytes (fat-storing cells, Ito cells). In the present study, we have utilized smooth muscle alpha actin antibody--a marker of myofibroblasts and smooth muscle cells--to study lipocytes in situ in normal and fibrotic rat liver as well as during their 'activation' in culture. Dual immunofluorescence studies on tissue sections from normal liver identified lipocytes as perisinusoidal desmin-positive, smooth muscle alpha actin-negative cells. In bile duct obstructed fibrotic liver, desmin-positive cells were numerous in areas of fibrosis and these cells also exhibited smooth muscle alpha actin. In carbon tetrachloride-induced fibrosis, cells expressing both desmin and smooth muscle alpha actin were present in fibrotic bands and in regenerating nodules. These results suggested that lipocytes had acquired characteristics of myofibroblasts during liver injury. To further address this issue we examined lipocytes immediately after isolation and also in primary culture. In freshly isolated lipocytes from normal liver, smooth muscle alpha actin was absent. In contrast, freshly isolated lipocytes from CCl4-treated animals expressed this smooth muscle marker immediately after isolation. In primary culture on plastic, lipocytes from normal liver began to express smooth muscle alpha actin coincident with culture-induced activation; at 14 days, smooth muscle alpha actin was identified in all cells. Electron microscopy demonstrated a highly developed array of microfilament bundles characteristic of actin filaments. Immunoblot of culture-activated lipocytes using the smooth muscle alpha actin antibody demonstrated the expected 42 kD protein (corresponding to the molecular size of smooth muscle alpha actin). Although smooth muscle alpha actin was readily detectable in culture-activated cells, it was not expressed in cells in which a quiescent phenotype was preserved by maintenance in culture on a laminin-rich gel. These findings demonstrate that the acquisition by lipocytes of a smooth muscle marker accompanies their 'activation', and are consistent with the hypothesis that lipocytes transform to myofibroblasts during liver injury.