The retinoblastoma (RB) and cyclin-dependent kinase inhibitor 2/multiple tumor suppressor gene 1 (CDKN2/MTS1) tumor suppressor genes play important roles in the regulation of the cell cycle. The protein products of these two genes, pRB and p16INK4A (“p16”), respectively, inhibit progression from G1 to S phase. Moreover, p16 has been shown to exert its function through inhibition of CDK4-mediated phosphorylation of pRB. Both genes have been found to be mutated or deleted in a wide range of primary human tumors and tumor cell lines. However, the presence of CDKN2/MTS1 containing nonneoplastic elements in every tumor specimen may contribute to the apparent lower deletion detection rate in resected neoplasms compared to cell lines. We have developed an immunohistochemical assay that allows us to assess p16 expression in formalin-fixed, paraffin-embedded tissues. As controls, we used paraffin-embedded pellets of cell lines with well-defined p16 status (four positive and four negative lines), as well as routinely processed nude mouse xenografts of two p16-positive cell lines. p16-negative cells were characterized by the absence of nuclear staining, whereas cytoplasmic staining was variable. In neoplastic and normal tissues, the level of p16 generally appeared to be low. We tested 75 random human mallgnancies from 4 different anatomic sites for p16 expression and correlated the findings with the immunohistochemical presence or absence of pRB. Twenty % of tumors selectively lacked pRB, while 37% of neoplasms had undetectable p16. In 43% of all carcinomas, both pRB and p16 could be detected. Significant differences existed in the expression of both tumor suppressor genes between carcinomas from different sites. Breast cancers had the highest rate of p16 negativity (13 of 20). Our data show that: (a) immunohistochemistry may be a suitable modality to screen for RB and CDKN2/MTS1 abnormalities in paraffin-embedded tissues; (b) undetectable levels of p16 expression occur at a relatively high frequency; (c) p16 and pRB expression in common human malignancies is not mutually exclusive; (d) loss of function of both tumor suppressor genes appears to be a distinctly uncommon phenomenon; and (e) different types of carcinomas have variable rates disturbance in the p16/pRB pathway.