High level expression of the cell surface adhesion molecule CD44 standard form and its splice variants, have been causally linked to tumor metastasis. In this study, we investigated the significance of CD44 expression in human prostatic carcinoma cells. Immunocytochemistry showed high level expression of CD44 in cells from a high grade prostate tumor, and two androgen-independent, invasive prostatic carcinoma lines, PC-3 and TSU-Pr1. Normal prostatic epithelial cells and LNCaP, a low metastatic, androgen sensitive cell line, expressed none to a very low level of CD44, although mRNA transcripts were detected in all cell lines. Immunoprecipitation detected two proteins of M(r) approximately 140 kDa and 210 kDa in PC-3, and predominantly the M(r) approximately 95 kDa protein in TSU Pr1, but none in LNCaP. Most importantly, a neutralizing antibody to CD44 inhibited cell proliferation and basement membrane invasive activity, suggesting a definitive role of CD44 in prostate tumor growth and metastasis.