1. The blood cytology of 41 pairs of virgin rabbits was studied, each pair consisting of a buck and a litter mate sister. A comparison of the values for each blood factor of the two groups was made by obtaining the mean difference and the standard error of the mean difference. In addition, correlation coefficients, both zero order and first order eliminating age, and regression equations were calculated for each of the blood cell elements of the bucks and does.
2. It was found that the male values for both the red cell count and the hemoglobin content were higher than the corresponding female values, and these differences were of statistical significance. No differences were observed between the two groups with respect to other cells. Significant positive correlations were obtained between the two sexes for the following blood constituents: Red blood cell count, white blood cell count, hemoglobin content, lymphocytes in both absolute and relative numbers, and neutrophils, basophils, and monocytes in relative numbers.
3. Since all known variables had been held constant, it was concluded that the higher red cell count and hemoglobin value for the male group were the results of constitutional differences between the sexes. Moreover, the significant correlations between the blood cells of male and female litter mates were ascribed to hereditary influences, and thus the female as well as the male rabbit is available for studies on the mechanism involved in the inheritance of the blood formula. In addition, since high correlations were obtained for cells which have been found to be the most reliable indices of the natural immunity of the male host to subsequent inoculation with tumor or syphilis, it was concluded that these cells should give dependable information concerning the natural resistance of the female host to these two diseases.