Microtubules and actin filaments are two of the major components of the cytoskeleton. There is accumulating evidence for interaction between the two networks. Both the ␣and -subunits of tubulin exist as numerous isotypes, some of which have been highly conserved in evolution. In an effort to better understand the functional significance of tubulin isotypes, we used a double immunofluorescence labeling technique to investigate the interactions between the tubulin -isotypes and the actin stress fiber network in cultured rat kidney mesangial cells, smooth-muscle-like cells from the renal glomerulus. Removal of the soluble cytoplasmic and nucleoplasmic proteins by detergent extraction caused the microtubule network to disappear while the stress fiber network was still present. In these extracted cells, the  I -and  II -tubulin isotypes were no longer present in the cytoplasm while the  IV -isotype co-localized with actin stress fibers. Co-localization between  IV -tubulin and actin stress fibers was also observed when the microtubule network was disrupted by the anti-tubulin drug colchicine and also by microinjection of the  IV -tubulin antibody. Our results suggest that the  IV isotype of tubulin may be involved in interactions between microtubules and actin. Cell