Two of the multiple forms of human guanylate kinase (the components e and c) described by MONN and CHRISTIANSEN could be separated from the guanylate kinase of Chinese hamster in man-Chinese hamster hybrids on cellulose acetate gel. By employing a number of enzyme markers, whose loci have been already assigned to particular human chromosomes, as main markers in man-Chinese hamster hybrids, evidence was gathered to indicate that the components e and c of GuK are determined by two separate loci situated on the human chromosome number 1 distal to the break point q 2.2.