4-Carbamoylimidazolium 5-olate (CIO), the aglycone of the nucleoside antibiotic, bredinin (4-carbamoyl-1-beta-D-ribofuranosylimidazolium 5-olate), exhibited potent cytotoxic effects of subclonal line F28-7 of C3H mouse mammary carcinoma FM3A cells in culture. We isolated 11 cell lines resistant to CIO from wild-type F28-7 cells mutagenized with N-methyl-N'-nitro-N-nitrosoguanidine. These resistant (cio') lines were 160- to 400-fold less sensitive to CIO than were the wild-type cells and inherited the resistant phenotypes during subculture for more than 3 months in the drug-free medium. They were cross-resistant to an adenine analog, 2,6-diaminopurine, while 2,6-diaminopurine-resistant (dap') lines, isolated independently, were cross-resistant to CIO. Neither of the cio' lines tested were able to form colonies in agar medium containing azaserine and adenine, nor were they able to incorporate tritiated adenine into the macromolecular fraction, indicating that they could not utilize exogenous adenine for growth. Enzyme assays using cell-free extracts revealed that all the cio' lines had undetectable levels of adenine phosphoribosyltransferase (EC 2.4.2.7) activity, but they, except one, had normal levels of hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8) and adenosine kinase (EC 2.7.1.20) activities. These results demonstrate that the CIO resistance in these lines is attributed to deficient adenine phosphoribosyltransferase activity and therefore that CIO is activated by adenine phosphoribosyltransferase to form a cytotoxic nucleotide within the drug-sensitive cells.