Cultured human lymphoblastoid cells of different lines elaborate rapidly pyrimidine-specific nucleosides when incubated as a dense population in a serum-containing milieu. No appreciable changes in viability or morphology were detected in the time required for maximal enzyme production. These nucleosidases degraded thymidine, uridine and cytidine; adenosine and guanosine were not catabolized. Cell viability and proliferation were not affected by these heat-labile and ultraviolet-sensitive enzymes. Products arising from nucleosidase degradation of radiolabeled pyrimidines, which were separated and identified by paper chromatography, were not incorporated by DNA- and RNA-synthesizing lymphoid or nonlymphoid cells.