A non-competitive enzyme immunoassay specific for delta aminolevulinate dehydrase has been devised and applied to rodent-human hybrid cell lines. Two different conditions have been used, one specific for the human enzyme and the other indicative of both rodent and human enzymes. The ratio of the values obtained under the two conditions was used to discriminate between positive and negative clones. By this method the gene for ALA dehydrase has been assigned to chromosome 9.