1762 mice of 2 genetically selected inbred strains (Prunt and 2 Prunt) were injd. subcut. with 1 mg. of methylcholanthrene at 60 days of age, developed fibrosarcomata and died. The mice were of both sexes and belonged to the l-15th litters. Their parents had belonged to 1 or 2 litters only for at least 10 generations. The number of mice developing tumors with the capacity to form discreet nodules in the abdomen by invasion or by metastasis was highest in mice of early litters and progressively diminished in mice of the succeeding litters. The slope of the decline of malignant fibrosarcomas was 5.5, a significant difference from slope "O". Mice of very advanced litters have never developed an invasive or metastasizing fibrosarcoma. Thus a characteristic of malignancy of the chemically induced fibrosarcoma is under the control of some mechanism which progressively changes during the life span of the parent and is handed down to the offspring.