Sixteen "A" strain mice were maintained, from the age of 3 months, on a Rockland rat diet supplemented with hard boiled eggs, one or 2 daily. Sixteen other mice of the same litters as the above, kept as controls, were maintained on the same Rockland rat diet, but without eggs. Of the latter group only 2 animals developed adenocarcinomas of the lungs, one at 20 and the other at 25 months of age. Of the 16 experimental mice, 12 developed malignant tumors. In 6 of them the tumors in each animal were limited to a single organ, while of the other 6 mice, which died at the age of 22-25 months, some had 2, others 3 and one had 4 primary malignancies. Adenocarcinoma of the lung was the most frequent malignancy (found in 10 of the experimental mice). Next in frequency were ovarian tumors, leukemias, mammary adenocarcinomas, an osteosarcuma and adenocarcinoma of the adrenal cortex. In many of these animals distant metastases were found. The liver seemed to be damaged in almost all of the experimental mice. Pathological changes were found in the ovaries of almost all the experimental females, while in all of the experimental males the testes appeared normal.