Recently a new proto-oncogene, the murine double-minute 2 (MDM2), has been described. MDM2 becomes oncogenic due to amplification and overexpression. Among other proto-oncogenes MDM2 becomes interesting since MDM2 protein can associate with both mutant and wild type p53 tumor suppressor gene products and thus inhibit p53-mediated transactivation of other genes. Loss of p53 tumor suppressor function is the most frequently observed alteration in human tumors. Immunohistochemical studies investigating the quantity of MDM2 protein in human sarcomas revealed an overexpression in 30% of the specimens. Here we describe the successful use of a monoclonal antibody (IF2) for the detection of MDM2 protein in paraffin-embedded tissue from human lung biopsies. 18 out of 44 specimens (41%), predominantly mucosal epithelial and glandular epithelial cells, stained positive for MDM2. No significant difference was observed between non-cancerogenic cells adjacent to tumor cells and those specimens without any tumor cells but altered by inflammatory processes. In general, the staining pattern was restricted not to the nuclei, but to selected subnuclear compartments, probably representing the golgi apparatus or the endoplasmatic reticulum. Our data support the hypothesis that in addition to its nuclear function of forming a complex with p53, MDM2 may also be secreted and thus have a transcellular effect.