The gene of the CD44 cell surface glycoprotein consists of 20 exons. Ten exons known as variant exons are inserted by alternative RNA splicing between exon 5 and 16, thus generating diversity in the extracellular portion of the protein. We have cloned the dog cDNA homologue of the human variant exon region of CD44 and characterized its transcript expression in normal lymphatic tissues. Using PCR with primers complementary to regions contiguous to the insertion point and a variant exon, all ten variant exons of dog CD44 were detected in a high number of different isoforms. Transcripts containing the ten variable exons directly spliced to both sides of the insertion point were detected. We found an extensive usage of all variant exons from v1 to v10 in a high number of different combinations, some presenting a tissue-specific expression pattern. A similar complex profile of transcript expression was found in peripheral blood lymphocytes from rat and human.