The v-fms encoded kinase, which is known to associate with cytoskeletal elements, was shown to phosphorylate histiocytic proteins that had physical and immunological attributes in common with tau. Tau is a microtubule associated protein that is aberrantly phosphorylated in Alzheimer's disease of the brain. Since c-fms expression is normally exclusive to cells of monocyte lineage, these cells were examined for expression of tau. Heat stable tau was identified in cultured peripheral blood monocytes. This represents the first molecular characterization of tau in cells of non-neural origin.