Sixty-seven galactose negative (Gal minus) clones were established from survivors after treatment of Chinese hamster lung (V79) cells with 5-bromodeoxyuridine and black light. The mutational origin of these clonal isolates was inferred on the basis of the previously demonstrated mutagenic action of the combined treatment, persistence of the Gal minus phenotype in cell progeny in the absence of selection, conditional lethality of three isolates, interallelic complementation, and assignment of a homologous human gene to chromosome A2. In contrast to the parental cells, Gal minus mutants could not utilize exogenous galactose, mannose, fructose, galactose 1-phosphate, glucose 1-phosphate, or glucose 6-phosphate. Permeation of galactose into mutant cells appeared unimpaired. In intra- and interspecific cell hybrids, the mutation causing the Gal minus phenotype behaved like a recessive character. The pleiotropic nutritional response was due not to deficiency of any one specific enzyme in the Leloir pathway but to significant reduction in activities of phosphoglucomutase, NADP-dependent isocitrate dehydrogenase, and perhaps other enzymes. In temperature-sensitive Gal minus mutants grown at permissive temperature or in complementing intra- and interspecific cell hybrids, the activities of these enzymes were restored to normal levels, accompanied by a regained ability to use the particular hexoses or hexose monophosphates. We postulate that the change from Gal + to Gal minus phenotype in hamster cells could be due to mutations at a regulatory gene locus, or at a yet unknown locus with enzymic defect that causes secondary metabolic imbalances.