1. A procedure for carrying out biological assays of adrenal cortical hormone by the mouse eosinophil method is described. 2. The test is sensitive to 1 μg of cortisone; there is a linear relationship between log dose and response with a stable variance; the slope and sensitivity of the test do not change significantly throughout the year. The test is, however, inaccurate (λ=0·56) in that a fourfold increase in dose is required for the mean responses of two groups of ten mice to differ significantly. 3. Five other biologically active steroids and two of their acetates have been compared. The 11-deoxysteroids were found to be inactive. The order of potencies were: cortisone (100), hydrocortisone (78), corticosterone (37), 11-dehydrocorticosterone (24). 4. There is no significant departure from parallelism between the curves for cortisone and cortical extract (Eucortone). 5. The test is not specific for adrenal steroids, as dog plasma protein gives a similar effect in adrenalectomized mice. Therefore, biological fluids containing protein must first be extracted with organic solvents before being tested for cortical hormone activity by this method. 6. No hormone was detected in the cells of adrenal venous blood. The hormone appeared to be in the plasma. 7. Hydrocortisone in small concentrations has no direct cytolytic action on eosinophils in vitro.