Quantitative transformation by carcinogens of Syrian golden hamster embryo cells from primary, secondary, or tertiary cultures could be obtained from cell pools frozen prior to any culturing with the same efficiency as with fresh cells. Cells retained the ability to activate a wide range of chemical carcinogens when the noncultured hamster cells were cooled at a controlled rate and when the established procedures for culturing cells were followed. The number of experiments that could be done with cells derived from frozen hamster pools could be increased by substitution of a hamster cell line for the feeder layer. The response to chemical carcinogens of different classes including carcinogenic hydrocarbons, aromatic amine derivatives, metal complexes, aflatoxin B1, N-methyl-N'-nitro-N-nitrosoguanidine, and UV irradiation was similar to that reported with fresh cells. Transformation rate increased with increasing carcinogen concentration. The factors important for the breeding of healthy animals and for selection of those appropriate for obtaining reproducible transformation experiments were enumerated.