We isolated several mutants that are highly resistant to 2'deoxy-2'-azidocytidine by culturing mutagenized mouse FM3A cells in a medium containing 10^-4 M of this analogue. The mutants incorporated deoxycytidine into acid-insoluble materials at a reduced rate, but incorporated cytidine normally. All the mutants isolated also were resistant to 1-β-D-arabinofuranosylcytosine. The activity of deoxycytidine kinase in the crude extracts of these mutants was reduced to about 10 % that of the FM3A cells. Ourresults indicate that 2'-deoxy-2'-azidocytidine is recognized as deoxyribonucleoside by deoxycytidine kinase in contrast to reports that phosphate forms of this nucleoside analogue are recognized as ribonucleotide by the ribonucleotide reductase from calf thymus and E. coli and by E. coli primase.