C-terminal src kinase (CSK) is a cytoplasmic protein which decreases activities of the Src family protein-tyrosine kinases. We produced a polyclonal antibody specific for CSK and analyzed the expression of CSK by immunoblotting in two human colorectal normal cell lines and eighteen cancer cell lines. CSK was detected in both the two normal and all the eighteen cancer cell lines. The expression of CSK obtained from human colorectal cancer cell lines was greater than that from human colorectal normal cell lines in most cases. The elevated expression of CSK in human colorectal cancer cell lines appeared to correspond to the elevated p60c-src (c-Src) and p62c-yes (c-Yes) protein-tyrosine kinase activities found in other studies. Thus, CSK may not have an anti-oncogenic role to play through the negative regulation of Src family kinases in colorectal carcinogenesis.