Cell form and cytoskeletal organization were investigated in 13 human breast carcinoma cell lines in vitro. Using tubulin antibodies and indirect immunofluorescence to detect the arrangement of cytoplasmic microtubules, three distinct cell phenotypes were recognized: (a) cells with extensive arrays of microtubules (type 1); (b) cells which were diffusely stained with microtubules apparent only near the cell margins (type II intermediate); and (c) cells in which individual microtubules could not be detected and only diffuse fluorescence was apparent (type II diffuse). Type I cells were flattened epithelial-like cells, much like normal mammary epithelial cells, which when stained with actin antibody displayed many brightly fluorescent parallel cables or "stress fibers." Many microtubules and microfilament bundles were observed in type I cells when examined by transmission electron microscopy. Type II cells were more rounded, often grew in multilayered colonies, and displayed fewer microtubules and microfilament bundles when examined by either immunofluorescence or electron microscopy. Type II cells ranged from very small rounded cells with diffuse tubulin and actin immunofluorescence (type II diffuse) to more flattened cells in which microtubules and actin cables were observed near the flattened cell margins (type II intermediate). Since all of the cells were derived initially from malignant metastatic lesions and some were tumorigenic when injected into athymic nude mice, we assume that they remained malignant in vitro. Thus, in human breast carcinoma cells in vitro, it is not possible to associate any specific cell morphology or cytoskeletal phenotype with cancer or metastasis in vivo. Whether or not these same conclusions hold for breast tumor cells in situ remains to be determined.