Recently galactosyltransferase has been noted as a new tumor marker. In this report galactosyltransferase activity was measured in sera of various diseases including malignant tumors and the levels of the enzyme activities were compared. The levels of the enzyme were higher in sera of malignant tumors and liver diseases than those in sera of other diseases. In malignant tumors little organ specificity was found with respect to the levels of galactosyltransferase activities. The levels of the enzyme were considered to have no relation to carcinoembryonic antigen or alpha-feto protein. On the other hand, in liver diseases the levels of the enzyme were correlated to the severity of liver damage. Therefore, it is reasonable to presume that the elevation of serum galactosyltransferase activity is caused chiefly by malignant tumors and liver damage.