Lck protein is expressed in some colon carcinoma cell lines but its expression in colon cancer cells in vivo has not been clarified. LCK transcription is regulated from two distinct promoters and initiated exclusively from the downstream promoter in colon carcinoma cell lines in contrast to peripheral lymphocytes. We investigated the expression of the downstream promoter-initiated LCK transcript in 18 colorectal primary cancer and normal mucosae, and two hepatic metastases, using a RNase protection assay with the EcoRI-BglII fragment of human LCK cDNA, YT16. In normal tissues, only traces of the LCK transcript were detected. The expression of the LCK transcript was augmented in 3/18 cancer specimens. The relative level of the LCK transcript in the cancer tissue compared to the average value of normal adjacent tissue was 10-60 in 3 cases, and 3-10 in 7 cases. One hepatic metastasis expressed more LCK message than the primary lesion. Our results indicate that the LCK message is strongly expressed in some colorectal cancers.