Male LAF1 mice, 10 hr after irradiation with 750 r, were injected intravenously with normal LAF1 spleen cells followed 1 day later by an intravenous injection of sheep red blood cells. The recipient mice were sacrificed 6 days after antigen injection and their spleens separated in vitro into two portions: a portion showing precursor cell activity and therefore containing a large number of plaque-forming cells (positive areas), and a portion showing no precursor cell activity and therefore containing very few plaque-forming cells (negative areas). The number of antibody-forming cells and precursor cells in each portion was measured. The ratio of plaque-forming cells to precursor cells was 32:1 in the positive areas and 5.4:1 in the negative areas. These data suggest that all precursor cells are not stimulated maximally by antigen.