A severe deficiency of a red cell pyrimidine 5'-nucleotidase was found to be associated with hereditary hemolytic anemia in four members of three kindreds. The syndrome was characterized by marked increases above normal in red cell basophilic stippling, total nucleotides, and GSH and by a fairly severe deficiency of ribosephosphate pyrophosphokinase (EC 2.7.6.1.). Patient erythrocytes uniquely contained large amounts of pyrimidine 5'-ribonucleotides. In earlier studies, these were erroneously considered to be adenosine phosphates, since all previous investigations of the nucleotides of human red cells and reticulocytes have shown 97% or more to contain adenine. Total nucleotides in patient cells were present in amounts 3-6 times greater than normal, and approximately 80% contained pyrimidine. The ultraviolet spectral curves of deproteinized red cell extracts exhibited a shift in maximum absorbance from the usual 256-257 nm to approximately 266-270 nm, and absorbance at 250, 270, 280, and 290 nm, expressed as a ratio of that at 260 nm, differed greatly from normal. The spectral characteristics of extracts provide the basis of a readily performed screening procedure, which does not require enzyme assay. The nucleotidase activity in deficient red cells assayed less than 14%, and usually less than 10%, of normal and much less in terms of reticulocyte-rich blood, where it was consistently found to be increased. The enzyme has a pH optimum of 7.5-8.0, is inhibited by EDTA, and does not utilize purine 5'-ribonucleotides or beta-glycerophosphate as substrates. While comparatively few family members have been available thus far for study, initial data are compatible with an autosomal, recessive mode of transmission of the deficiency. The pyrimidine 5'-ribonucleotides are presumably derived from RNA degradation and, not being diffusible, accumulate when the enzyme catalyzing their dephosphorylation is deficient. It is postulated that the prominent basophilic stippling results from retarded ribosomal RNA degradation secondary to accumulation of degradation products, namely pyrimidine 5'-ribonucleotides. Ribosephosphate pyrophosphokinase deficiency is considered to be an epiphenomenon. The mechanism responsible for increased red cell GSH is unknown.