Previous attempts to transform human foreskin fibroblasts in vitro with N‐meihylnitrosourea (MNU) or N‐ethylnitrosourea (ENU) have been unsuccessful, and concurrent treatment with cocarcinogens or tumor promotors and either MNU or ENU have also failed to produce a neoplastic response. The present study was undertaken to test the effect of sodium saccharin on MNU‐ or ENU‐induced cell transformation. Saccharin alone was not effective in inducing the growth of colonies in soft agar (anchorage‐independent growth). However, concurrent treatment with saccharin (50 μg/ml, nontoxic dose) and MNU or ENU (29 μg/ml or 44 μg/ml, respectively) was effective in inducing transformation (greater than 300 colonies/10⁵ cells), but only when the cells were treated with saccharin after being released from a G₇ block (amino acid deprivation) and followed by MNU or ENU treatment in early S phase. In contrast to results obtained with other chemical carcinogens, transformation frequencies induced by saccharin and MNU or ENU were only slightly decreased in the absence of insulin, which is normally required for growth in this system. Saccharin‐MNU‐ or saccharin‐ENU‐treated cells that exhibited growth in soft agar also exhibited cellular invasiveness in 9‐d‐old embryonic chick skin in vitro. In addition, these cells reacted with a monoclonal antibody prepared against a molecular weight 115,000 sarcoma‐cell surface‐associated glycoprotein and also developed tumors in nude mice. These data demonstrate the cell‐cycle‐dependent cocarcinogenic potential of saccharin and MNU or ENU in cultured human skin fibroblasts.