Variant isoforms of CD44, a family of cell-surface glycoproteins generated by alternative splicing and post-translational modifications, are expressed in a variety of human tumors and play important roles in tumor progression and metastasis formation. The murine monoclonal IgG1 antibody VFF18, specific for an epitope encoded by human CD44 variant exon 6, binds with high affinity to the recombinant protein (Kd = 1.7 x 10(-10) M) as well as to tumor cell lines in vitro, and is suitable for immunohistochemical analysis of human tumors. Screening of more than 500 tumor samples of different histogenesis showed that VFF18 most strongly and uniformly reacts with squamous cell carcinomas (SCC). Detailed analysis of 185 SCC (head and neck, lung, skin) confirmed reactivity of the antibody with 99% of the samples, with intense and homogeneous staining of the tumor cells in the majority of cases, whereas reactivity of VFF18 with normal tissues is limited to certain epithelia and activated lymphocytes. When radiolabelled VFF18 was administered to nude mice bearing human epidermoid carcinoma (A-431) xenograft, fast and selective tumor uptake of the radioimmunoconjugate with a maximum of 18% of the injected dose per gram of tissue was observed. Taken together, these data suggest that mAb VFF18 is a promising targeting vehicle for radioimmunotherapy of squamous cell carcinomas in humans.