Eight persons who had no activity of uridine diphosphate galactose 4'-epimerase in their circulating blood cells are known today. They were healthy members of three different families. Propositi were newborns discovered in a mass screening test for blood galactose which registered high levels of erythrocyte galactose-1-phosphate. Epimerase deficiency was restricted to circulating blood cells, but in liver biopsy specimens, in cultured skin fibroblasts and in activated lymphocytes epimerase was found to be normally active. Heterozygotes had intermediate red cell epimerase activity. There were no symptoms of galactose intolerance and no pathology related to the enzyme defect. All 8 epimerase deficient persons had a ccdee Rhesus genotype. Attempts at demonstrating genetic linkage between the epimerase and Rhesus loci were unsuccessful because of the difficulty encountered in ascertaining epimerase heterozygosity. Individuals with hereditary UDP-galactose 4'-epimerase deficiency appear to produce an unstable mutant enzyme requiring higher NAD concentration for maximum activity.