In two strains of fibroblasts originating from fragments of myocardium from a 5 day old rat of the Slonaker-Addis strain, a strain known to develop tumors but rarely, abnormal morphologic alterations took place which eventuated in neoplastic changes. This happened only after repeated exposure of the cultures to an atmosphere of nitrogen (anaerobiosis) and did not occur in the control cultures of fibroblasts originating from the same rat heart, and grown continuously throughout the same period (2 ½ years) without exposure to nitrogen.

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