A cytoplasmic form of thymidine kinase was detected in sera from rats bearing Morris hepatoma 7777. Polyacrylamide electrophoretic properties demonstrate similarity between the serum enzyme and the same enzyme extracted from hepatoma tissue. This form of thymidine kinase was also described in normal fetal tissue and a variety of human tumor cells. Serum isoenzymes of thymidine kinase may be a useful adjunctive test for the presence of certain types of malignant tumors.