This study was carried out to investigate the effects of nucleic acid related compounds and metal ions on activities of cytidine deaminase from Bacillus subtilis ED 213. The purified cytidine deaminase was weakly inhibited by 1mM GMP, IMP and ATP, but not affected by other nucleic acid related compounds such as CMP and UDP. The apparent Km values for cytidine, deoxycytidine, 5 methylcy tidine, fluorodeoxycytidine, and 5 bromocytidine were calculated to be 6.6$ imes$10-4M, 6.0$ imes$10-4M, 0.9$ imes$10-4M, 0.8$ imes$10-4M, and 2.0$ imes$10-3M, respectively. The cytidine deaminase was completely inhibited by 1mM Hg2＋, and mildly inhibited over 40% by metal ions such as Na＋ and Fe2＋. However the enzyme activity was activated more than 40% by 1mM Mg2＋.