1. Data have been presented, showing that it is possible to standardize conditions under which absolute eosinophil levels in the tail blood of mature male mice characteristic for different strains can be obtained.
2. The highest eosinophil levels were found in the strain of mice with the lowest susceptibility to mammary cancer among the stocks tested.
3. The absence of the mouse mammary cancer milk agent did not affect the characteristic eosinophil morning level in A or Z mice.
4. Emphasis is placed upon the necessity of rigid control of the circumstances of observation if reproduceable results are to be obtained in the study of eosinophil levels.