We have compared the levels of creatine kinase (CK) activity and the distribution of CK isoenzymes determined by agarose gel electrophoresis in normal colon, liver and lung tissues, and in colon, liver and lung adenocarcinomas, lung squamous cell carcinomas and lung carcinoids. Colon and lung adenocarcinomas, and squamous cell carcinomas presented lower CK activity than the normal tissues and no differences were found between hepatocarcinoma and normal liver tissue. In contrast, lung carcinoids had higher CK activity than normal lung tissue. Type BB-CK was the predominant isoenzyme in normal lung, colon and liver tissues. Type MM isoenzyme was detected in normal lung and type MB-CK was found in normal colon. In most lung tumours the CK isoenzyme electrophoretic pattern did not change. However, no type BB-CK was detected in some hepatocarcinomas, type MM-CK decreased in lung carcinoids and type MB isoenzyme was not observed in colon adenocarcinomas. It is concluded that in most tumours there is a decrease in the expression of type B- and type M-CK subunits, whereas in lung carcinoid the expression of type B-CK activity increases. Thus, the increase in type BB-CK observed in the serum of patients with lung and colon adenocarcinomas is probably due mainly to enhanced enzyme release as a result of tumour cell necrosis.