The ontogeny of IgD was examined in neonatal to adult mice by immunofluorescence. The first δ‐positive cells appeared in the spleen at about 3 days of age, while lymph nodes from 4 to 5‐day‐old mice already contained 10–20% δ‐positive B cells. In order to study the function of IgD, we developed a system in which the appearance of δ‐positive cells was suppressed by treatment of mice from birth with an anti‐δ allotype serum. Immunofluorescence studies showed that very few δ‐positive cells could be found in the spleen and lymph nodes of such suppressed mice, and also that the percentage of μ‐positive cells was reduced by about 50%. The recovery from suppression was rapid, indicating that potential δ‐positive cells were modulated rather than killed. Studies of the immune response of suppressed mice in vitro and in vivo showed that suppression had little effect on the IgM response but caused a definite reduction in the IgG response.