Methods and Results

The first experiments were designed to produce isologous immunity against MCA-induced sarcomas in a number of inbred mouse strains or their F1 hybrids. The animals were usually 3 to 6 months of age; both sexes were used. They were obtained from 3 sources: the animal-production facilities at the National Institutes of Health, the animal colonies of Dr. W. E. Heston, or those of Dr. H. B. Andervont. The tumors were induced by subcutaneous implantation of a few crystals of MCA or of pellets of paraffin or cholesterol (2 to 3 mm. in diameter) that contained 10 or 5 percent MCA, respectively. Histologically, all were fibrosarcomas of similar type. Fourteen such tumors were produced and used. When a tumor arose it was excised, and a small fragment was transplanted with a 12-gauge trocar to the subcutaneous tissues of the dorsum of mice of the isologous strain. The growths resulting from this immunizing first transplant often did very poorly but were allowed, in those animals in which growth occurred, to attain a size of approximately 1 em. At that time, each first-transplantgeneration tumor was strangled by successively tighter ligatures, following closely the technique used by Foley (3). In later experiments of this series, simple surgical excision was substituted for ligation without apparently altering the results." Two days to 2 weeks after final eradication of the first-transplant-generation tumors, each animal, including those in which the tumor had not grown, was tested for tumor immunity by subcutaneously implanting a second fragment of tumor by trocar.(The time interval before implantation of the second fragment of tumor, within the limits employed, was not a significant variable.) The second, or chal-