Cryopreserved primary cultures of golden Syrian hamster embryo cells were used as the source of target and feeder cells for establishing an in vitro carcinogenesis bioassay. The primary culture giving the best overall response in a pretest before freezing gave positive results in 20 consecutive experiments when retested with 3-methylcholanthrene after cryopreservation, indicating that pretested cryopreserved cultures can serve as a source of susceptible target cells in an in vitro carcinogenesis bioassay. Similarly prepared and cryopreserved cultures served satisfactorily as feeder cells. Susceptible positive cultures were used to test a large number of carcinogenic and non-carcinogenic chemicals in this system. The results showed a very high positive correlation (90.8%) between morphological transformation and the reported carcinogenic activity of the chemicals. Transformation was not observed when cells were tested with a few carcinogens that may not be metabolized to their active forms by early passage hamster embryo cells. N-2-acetylaminofluorene transformed cells only when tested in the presence of hamster liver microsomes. No false positive results were obtained when non-carcinogens were bioassayed, nor was spontaneous transformation observed in control cultures treated with medium alone, 0.2% dimethylsulfoxide or other solvents. Cultures derived from morphologically transformed colonies arising after treatment of cells with several known carcinogens were tumorigenic in vivo, confirming the correlation of morphological transformation with tumorigenicity and the validity of altered morphology as an in vitro criterion for carcinogenicity in vivo.

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