Human mammary epithelial cells (HMEC) were immortalized by serial passaging through senescence (M0) and subsequent transduction with the catalytic subunit of the human telomerase gene (hTERT). These cells acquired multiple non-random cytogenetic abnormalities with lengthy passaging in vitro, but are still not tumorigenic in irradiated nude mice and cannot grow in soft agar. Transduction, of late passage immortal HMEC from a single donor, with a retroviral vector containing the mutant autoactive H-Ras 61L gene, enabled immortal HMEC to acquire anchorage independent growth properties. Three colonies were picked and all three were found to be tumorigenic. One colony exclusively produced epithelial tumors in nude mice, but the other two colonies gave rise exclusively to malignancies in which the cells displayed a spindle morphology. In this paper we describe the characteristics of the tumors arising from one of these 'spindle colonies'. These tumors were strongly positive for vimentin staining and virtually negative for pan-cytokeratin staining, on immunohistochemistry. Cytogenetic analysis of the cells derived from these tumors confirmed that they were derived from the original cultured, immortalized mammary cells. We conclude that the HMEC have undergone metaplastic transformation due to the high levels of H-Ras 61L and telomerase activity that they display, and the derived tumors are best described as spindle cell carcinomas.