p16^INK4a and p15^INK4b are cell cycle regulators that specifically bind to and inhibit the cyclin D-dependent kinases, cdk4 and cdk6. Because these genes undergo frequent deletions and/or mutations in various human cancers, we examined the status and expression of the cognate mouse cdk inhibitors in a panel of 29 cell lines, as well as in 12 primary tumors, representing different stages of mouse skin carcinogenesis. Deletion of p16^INK4a and/or p15^INK4b was seen in 8 of 10 cell lines derived from spindle carcinomas, the most advanced stage of skin carcinogenesis. Five showed deletion of both genes, and three had independent deletions of p16^INK4a or p15^INK4b, but in those retaining p16^INK4a, expression of the protein was not detected. By contrast, none of 19 more differentiated squamous cell lines exhibited such deletions. In several cases, primary tumor DNA was available, and two spindle tumors showed the same deletion pattern as observed in the corresponding cell lines. In apparent contrast, comparison of two clonally related squamous and spindle cell lines derived from a single carcinoma showed unusually high levels of p16^INK4a and p15^INK4b only in the invasive spindle cells. Therefore, deletion or altered regulation of p16^INK4a and p15^INK4b occur concomitantly with the loss of differentiation associated with the late spindle stage of tumor progression in mouse skin.