Culture of human lymphoblasts in medium containing limited glutamine(less than 0.2 millimolar) effects an inhibition of purine base reutilization and synthesis de novo. Both processes appear to be inhibited by a decreased availability of the common substrate 5-phosphoribosyl-J-pyrophosphate. Under conditions of glutamine limitation, the maintenance of capacity for purine base utilization and synthesis de novo and consequently growth was dependent on supplementation of cultures with inosine. The purine nucleoside provides a source of ribose phosphate for synthesis of 5-phosphoribosyl-J-pyrophosphate. Glutamine limitation may result in a decreased availability of ribose-5-phosphate.