Fourteen spontaneous tumors of an inbred strain of guinea pigs have been described. No tumors were found in guinea pigs under 3 years of age, thus supporting the observation of Papanicolaou and Olcott (43) that spontaneous tumors occur rarely in guinea pigs which do not survive to senility. While this constitutes an incidence of only 2.4 per cent of the total population of this colony, it represents an incidence of 14.4 per cent of the animals surviving 3 years or longer.

These fourteen tumors bring the total number of reported spontaneous neoplasms in guinea pigs to 138. The latter have been tabulated as to site of origin and microscopic characteristics, and they are reported for the convenience of investigators encountering additional instances of spontaneous tumors or as a frame of reference for those studying the induction of tumors in guinea pigs.

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This work was supported by Research Grant #C-1590 (Cl-6) from the National Cancer Institute of the National Institutes of Health, Department of Health, Education, and Welfare.

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