In Chinese hamster ovary cells, two different types of mutants resistant to purine nucleoside analogs have been isolated. One type of mutants selected in presence of C-nucleosides formycin A and formycin B (FomR mutants) exhibited a high degree of cross resistance to different C-nucleosides but showed very slight to no cross resistance to various N-nucleosides. In contrast, mutants selected in presence of toyocamycin (Toyr mutants) exhibited a high degree of cross resistance to all C- and N-adenosine analogs. Studies on the cellular uptake and phosphorylation of [3H]adenosine, [3H]tubercidin, and [3H]formycin A show that, unlike the Toyr mutants which show reduced phosphorylation of all three compounds, the FomR mutants show reduced cellular phosphorylation of only [3H]formycin A. It is suggested that the genetic lesion in the FomR mutants affects adenosine kinase in a novel manner that specifically affects the phosphorylation of C-purine nucleosides.