An inherited deficiency of uridine-5'-monophosphate synthase has recently been discovered in dairy cattle. The possibility was explored that this deficiency might be detected by low amounts of erythrocyte 3-N-ribosyluric acid, since it is synthesized by uridine monophosphate synthase. Ribosyluric acid was lower in six dairy cows with the deficiency compared to six controls, paired on the basis of age, stage of lactation, and diet (.66 vs. 1.38 mumol/ml). However, ribosyluric acid concentration measured in 50 normal cows showed a wide range (.28 to 2.35 mumol/ml), with nine lower than the mean for the deficient animals. This extensive variability may limit the usefulness of ribosyluric acid in diagnosing the inherited deficiency of uridine monophosphate synthase.