Cytidine deaminase (CDD) catalyzes the deamination of various cytosine nucleosides to their corresponding uracil nucleosides. Due to its ability to inactivate cytidine analogs, it has been a target of considerable interests. The levels of CDD differ greatly among the various organisms studied, in some organisms and cell types CDD appears to be absent. In Enterobacteriaceae cytidine and deoxycytidine are used as carbon source, through the intermediate formation of uridine and deoxyuridine respectively. It is the pentose moiety which is catabolized. Uracil is utilized as pyrimidine source and excess uracil is excreted. When nucleosides are added to growing cells the synthesis of the nucleoside catabolizing enzymes is induced. The role of CDD in mammalian cells has been less well defined. The activity of CDD has been measured in human white blood cells, leukemic cells and in various cell lines. Low enzyme levels have been found in immature cells. An inverse relationship with respect to adenosine deaminase levels is seen, indicating the absence of a coordinate regulation of purine and pyrimidine nucleoside deamination. The determination of CDD levels might prove useful in studies to identify maturating steps of human blood cells.