A technique involving culture in soft agar was used for the assay of forward mutation of V79 cells to 6-thioguanine (6TG) resistance. The main reason for the use of soft agar was to prevent reduction in recovery of mutants depending on the cell density plated for mutation selection, which is the chief problem in the liquid method, and which results mainly from metabolic co-operation due to cell-to-cell contact. V79 cells grew well in fortified soft agar medium (DMEM + 20% FBS) showing cloning efficiencies (greater than 80%) as high as in liquid culture. Therefore, V79/HGPRT mutagenesis could be assayed quantitatively in soft agar culture. The frequency of 6TG-resistant colonies in agar selective medium increased linearly with increase in concentration of EMS. Toxicity and mutagenic responses were greater in soft agar than in liquid culture. In cultures of untreated and EMS-treated cells, more than 95% of the 6TG-resistant colonies isolated were aminopterin-sensitive. Use of soft agar for selection prevented the reduction in the number of mutants with increase in the size of inocula on plating up to 1-2 X 10(6) cells per 9-cm dish: in liquid culture, even with a lower plating number (2 X 10(5) cells per 9-cm dish), a notable reduction in numbers of mutants was observed. This character was re-examined in a reconstruction experiment. The results show that, when up to 2 X 10(6) cells were plated per 9-cm dish, 6TG-resistant cells were almost completely recovered from the soft agar medium, whereas only 10% were recovered from liquid culture.