To determine the extent and nature of individual variation in the development of specific antigen-binding cells, the numbers of cells specific for each of two antigens in the spleens of individual random-bred Swiss-L and inbred CBA/J and BALB/c fetal mice were measured as a function of spleen size. For Swiss-L fetuses, the ratio of antigen-binding cells to nucleateated cells varied more than would arise from sampling fluctuation. For each inbred strain, however, the number of cells specific for a given antigen was a constant proportion of the total number of nucleated cells within sampling error. These proportions varied from antigen to antigen, and from strain to strain. The ratio of the proportions of cells specific for the two antigens, however, differed no more from CBA/J to BALB/c mice than would be expected in repeated samples of cells from the spleen of a single fetus. These results confirm at the level of the individual fetus the uniform pattern of development seen for populations of fetuses. They reveal a surprising precision in the proliferation of specific antigen-binding cell populations and suggest that the development of these cells may be subject to strong genetic controls.