Abstract: Five strains of Streptomyces were screened for the abilities of their extracts to catalyze the hydrolytic or deaminating activities of purine and pyrimidine ribonucleosides and their bases. These studies are rare in Streptomyces. No hydrolytic cleavage for N-glycosidic bond of nucleosides was observed in all screened strains. Hydrolytic deamination was the only degradative activity occurred with cytidine (as substrate) from the ribonucleosides and their bases tested. Streptomyces hygroscopicus NRRL B-1476 gave the highest level of the hydrolytic deamination of cytidine to uridine. Uridine was chromatographically identified in cell-free extracts. Optimum pH and temperature of the enzyme activity were determined at 7.0 and 50°C, respectively. Thermal stability experiments indicated that the enzyme completely restored its activity at 50°C for 30 min, however a complete loss in enzyme activity was recorded when the enzyme was incubated at 80 and 90°C for 20 and 5 min, respectively. Dialyzed extract caused an increase in enzyme activity of about 55%. Results obtained indicate the involvement of sulfhydryl group(s) in the catalytic site of the enzyme. HgCl2, CuSO4 and FeCl3 (10-2 M) caused a complete inhibition of enzyme activity, whereas, little enzyme activity was retained in presence of AgCl2, MgSO4, BaCl2 and NaCl. Inhibition by uridine was of the competitive type and the enzyme exhibited classic Michaelis Menten saturation kinetics. Its apparent Km and Ki values were found to be 4.16 and 21.9 mM, respectively.