To differentiate cultured rat liver myofibroblasts, fat-storing cells, aortic smooth muscle cells and skin fibroblasts from each other, desmin and vimentin stainings were undertaken by indirect immunofluorescence using monoclonal antibodies. In myofibroblasts, the reaction with antibodies to vimentin was positive but that with antibodies to desmin was virtually negative. In primary cultures as well as subsequent passage of fat-storing cells, reactions with antibodies to both desmin and vimentin were positive. In primary culture of smooth muscle cells, both reactions were positive, but in the first passage, smooth muscle cells lost the reactivity with antibodies to desmin. Fibroblasts showed a positive reaction with antibodies to vimentin and a negative one with antibodies to desmin. Thus, immunohistochemistry of intermediate filaments allows for the differentiation between fat-storing cells, which are desmin- and vimentin-positive, and myofibroblasts or fibroblasts, which are desmin-negative but vimentin-positive. Smooth muscle cells are also vimentin-positive and become desmin-negative after the first passage.