A composition and method is provided for modifying sperm fertility in mammals for the purpose of biasing the gender in favor of male offspring. The composition includes an amount of a class of compounds of phenoxazine or phenothiazine, having the structure (FIG. 1) and ionic derivatives thereof, wherein R1 is H, a lower alkyl group or N(R3)2, R2 is N, S, O, and R3 is H or a lower alkyl group. A second component of the composition is an amount of a hexose sugar or the phosphorylated hexose esters of such. When mixed with living sperm prior to or at the time of insemination, the composition results in modified rates of conception and an alteration of the birth sex ratio.