Precursor analysis of the function of murine B cells with surface IgD has been accomplished using an alloantiserum directed against a δ allotype and cellular affinity chromatography. Spleen cells from 3‐week‐old C57BL.Ige mice were treated with a TNP‐labeled anti‐δ allotype serum, washed and passed through an anti‐TNP affinity column. The cells passing such a column were depleted of IgD‐bearing lymphocytes by immunofluorescence and also showed a greatly reduced ability to develop into specific antibody‐forming cell clones vs. the fluorescein (FL) hapten in a limiting dilution microculture system in vitro. In contrast, the IgD‐bearing cells eluted from such columns were slightly enriched in precursor activity compared to the original population. Moreover, brief treatment with anti‐δ serum alone produced a partial reduction in the precursor frequency vs. FL with C57BL.Ige spleen cells (which bear the appropriate δ allotype) but not C57BL/6 spleen cells (which are negative for this δ allotype). This effect varied with the age of the spleen donor, but was most dramatic with splenocytes from 2 to 3‐week‐old mice. This contrasts with the effect of anti‐pi pretreatment, which only inhibited neonatal precursors. The results are interpreted in terms of the maturation of murine B cells and the function of IgD and IgM receptors in triggering an immune response.