OBJECTIVE: The functional consequences of reintroduction of the CD44H cell adhesion molecule into colon carcinomas were investigated. BACKGROUND: CD44 is a cell surface adhesion molecule that is normally present in numerous isoforms as a result of messenger RNA alternative splicing. Individual CD44 isoforms differ in their ability to enhance tumorigenic or metastatic potential when overexpressed on tumor cells. Reverse transcriptase-polymerase chain reaction analysis demonstrates that CD44H is down-regulated during transformation of normal colon mucosa to carcinoma. The functional consequences of CD44H down-regulation in colon carcinomas has not been clarified. METHODS: Tumor cell lines and fresh tissue specimens were examined for CD44 expression by Western blot analysis. CD44H cDNA and site-directed mutants of CD44H cDNA were transfected into colon carcinoma cells. Stable transfectants were examined for adhesion to hyaluronate, in vitro growth, and in vivo growth. RESULTS: CD44H expression was nearly undetectable in primary colon carcinomas and colon carcinoma cell lines. In contrast, normal mucosa expressed high levels of CD44H. When CD44H was reintroduced into colon carcinoma cells, their in vitro and in vivo growth was significantly reduced. This CD44H-mediated growth rate reduction required an intact cytoplasmic domain. CONCLUSIONS: Transformation of normal mucosa to colon carcinoma is associated with a down-regulation of CD44H, which consequently may enhance the growth rate and tumorigenicity