The c-src proto-oncogene has been implicated in the progression of primary human colorectal carcinoma to hepatic metastasis. To determine if increased pp60c-src tyrosine kinase activity is a colon-specific phenomenon present in colorectal metastases to all sites, the pp60c-src-specific kinase activity of noncolon tumor metastases to the liver was compared to that of colorectal liver metastases. Activity of extrahepatic colon carcinoma metastases was compared to that of colorectal liver metastases as well as that of normal colonic mucosa. The specific activity of pp60c-src in multiple synchronous metastases from colon carcinoma was also examined. Tyrosine kinase activity was determined by immune complex kinase assay; protein levels were determined by immunoblotting. Specific activity was calculated for each group by dividing the total activity by protein level. Colon carcinoma metastases to the liver had significantly (P < 0.04) increased pp60c-src activity with an average 2.2-fold increase over normal mucosa. In contrast, noncolon tumor metastases to the liver showed minimal pp60c-src kinase activity. Extrahepatic colorectal metastases demonstrated significantly increased (P < 0.005) pp60c-src activity with an average 12.7-fold increase over normal mucosa. When compared to colon liver metastases, extrahepatic colorectal tumor metastases show a significant difference in activity (P < 0.05) with an average 5.7-fold increase. Examination of multiple synchronous colon carcinoma metastases confirmed these results. In summary, we conclude that (1) the activation of pp60c-src between primary tumors and metastases is specific to colon metastases, and (2) although pp60c-src activity is significantly increased in colorectal metastases, site-specific differences in the magnitude of activity are evident.