We have examined the expression of the transmembrane glycoproteins CD44 in four human prostate tumor cell lines. Expression was examined at the protein level by flow cytometric analysis and Western blot, and at the mRNA level by reverse transcription-polymerase chain reaction (RT-PCR). All four cell lines (DU145, LNCaP, PC3, and ND1) expressed the standard CD44 isoform (CD44s) at the mRNA level and all cell lines except LNCaP expressed CD44s at the protein level. All four cell lines contained one or more isoforms containing the v6 region (exon 10) at the mRNA level, which has been associated with metastatic potential. However, a subpopulation of LNCaP and ND1 cells showed protein expression of v6. In addition, soluble CD44 isoforms were identified in cultured supernatants from all cell lines except LNCaP. These results show that CD44 isoforms are expressed on human prostate tumor cell lines, including the expression of variant isoforms containing the v6 region, and provide a rationale for the further study of this cellular adhesion molecule in prostate cancer. In addition, preliminary results indicate altered expression of CD44 in human prostatic adenocarcinomas examined immunohistochemically.