[en] Many variables (e.g. cell inoculum size, mutagen dose, expression time, and concentration of the selective agent) are known to affect the induced mutation frequency obtained in cultured mammalian cells. The authors have studied the effects of several parameters on the frequency of radiation-induced resistance to 8-azaguanine in asynchronous V79-171B hamster cells. Inoculation with 10⁵ cells was followed by graded doses of radiation, expression times were optimized to maximize mutation frequency, and then the treated cells were challenged with 8-azaguanine for ten days. The optimal expression times which maximized mutation frequency were dose dependent and are in the range of 14-24, 24, and 24-36 hours respectively for doses of 250, 40 and 800 rads. A time interval of 24 hours between two 250-rad fractions resulted in a mutation frequency smaller than that obtained from administration of a single 500-rad dose. With 36 hours between halves of the dose, the induced mutation frequency was an order of magnitude lower than that produced by a single dose and actually below the unirradiated (spontaneous) frequency. Maintenance of cells after irradation first at 18⁰C for 24 hours, and then allowance of expression at 37⁰C for 24 hours, increased both the spontaneous and induced mutation frequency. A one-hour postirradiation balanced salt-solution treatment did not affect the number of spontaneous mutants that arose, but reduced the number of induced mutants. Thus, the balanced salt treatment lowers the induced mutation frequency about a factor of two. The possible significance of these results are discussed with respect to the role of radiation repair mechanisms during mutagenesis, and to recovery at low dose rates. A working hypothesis is advanced to explain the possible mechanism which causes expression time to vary as a function of the dose of mutagen. (author)