Hybrids have been developed from two cell lines originally derived from human mammary epithelial cells. MTSV1-7 (hygro), developed from cultured milk epithelial cells, shows a cuboidal morphology, expresses high levels of keratins (but no vimentin), and forms ball-like three-dimensional structures in collagen gels. 5.3.1E (neo), derived from a cell cultured from tissue taken from a primary breast cancer, shows an elongated morphology, expresses high levels of vimentin and low levels of keratins, and does not form structures on collagen gels. An examination of the hybrids formed by fusion of MTSV1-7 cells and 5.3.1E cells showed that while all could form three-dimensional structures in collagen gels, the type of structures formed resembled either ductal or alveolar-ball-like structures depending on the individual hybrid. Nine hybrids were examined and a clear correlation was observed between cell shape as seen on plastic, intermediate filament expression, and the form of the structures as seen in collagen gels. All the hybrids resembling the parent MTSV1-7 cells by showing a cuboidal morphology and high level of keratin expression formed ball-like structures; on the other hand, hybrids resembling 5.3.1E by showing an elongated morphology and a high level of vimentin expression formed duct-like structures in collagen gels. The results show that the ability to form structures in collagen gels was inherited in a dominant fashion from the MTSV1-7 parent. They also suggest that the profile of intermediate filament expression in the hybrids may influence both cell shape on plastic and the morphology of the three-dimensional structures formed in collagen gels.