We have investigated the functional integrity of the retinoblastoma tumor suppressor pathway in five human squamous cell carcinoma lines. Elevated activity of cyclin-dependent kinase 6 (cdk6), a pRB kinase, was detected in all five squamous cell carcinoma lines. Overexpression of the cdk6 protein was detected in one of the five cell lines. The cdk6-specific inhibitor p18ink4C is expressed and associated with cdk6 in all five squamous cell carcinoma lines. In contrast, only very low levels of p16ink4A were detected in these cell lines. This may contribute to the elevated activity of cdk6 in these lines. Elevated activity of cdk6 may result in hyperphosphorylation of the retinoblastoma protein and, therefore, compromise its negative growth-regulatory activity.