CD44 cell-surface receptor expresses multiple isoforms, some of which are believed to play a role in tumor growth and metastasis. The CD44 gene is composed of 19 exons, of which 9 (exons 6 to 14) are alternatively spliced to form inclusions in the intervening membrane proximal region. Sequences present in the shortest metastatic variant cloned from a rat metastatic cell line have been shown to correspond to human exons 10 and 11, also called exons v6 and v7. Using RT-PCR, we have addressed in detail the CD44 isoforms produced in human breast and colon tumors. We analyzed 53 breast-tumor- and 58 colon-tumor-related samples as well as 1 benign mastopathy, 1 normal breast, 4 non-invaded lymph nodes and 8 normal colon tissues. All tumors analyzed expressed the hemopoietic CD44 (CD44H) isoform (no alternatively spliced exons added), whereas 81% expressed the CD44E form (addition exons 12, 13 and 14). Furthermore, 85% of tumors presented complex patterns of expression, with an average number of 5 to 6 bands detected. In view of their implication in the metastatic process, we investigated in greater detail the isoforms containing exons 10 and 11 (v6 and v7). Exon 10 was more frequently expressed than exon 11, 80% and 57% of the samples respectively. The great majority of cases showed ladder-like patterns starting from the shortest forms (exons 5-10 or 5-10-11) and larger-molecular-weight bands corresponding predominantly to sequential inclusions of exons from 3' to 5'. Exon-10 and exon-11 variants were also found in one benign mastopathy. The majority of normal tissues (1 breast and 6/8 colon) expressed only the CD44H isoform. These data indicate that expression of metastatic variants is common in human breast and colon tumors and can occur early during cancer progression, as testified by their presence in a benign breast tumor. While expression of exon-10 variants were correlated with presence of distal metastases in colon tumors, exon-11 variants were not (metastatic events were too rare in our breast-tumor series to reach significance). This suggest that exon 10 may correspond to the minimal sequences required to favor metastatic events.