Whether recently generated peripheral B cells in adults are functionally equivalent to immature B cells in the neonatal spleen is unknown. We have identified a splenic B cell subpopulation in adults whose phenotypic and in vitro characteristics closely resemble those of neonatal B cells. These cells are defined by the cell surface phenotype heat-stable Aghi (HSAhi), and make up 10 to 15% of the adult splenic B cell pool. HSAhi B cells in adults bear the immature phenotype B220lo sIgMhi, and are 50% sIgD+. Furthermore, after sublethal irradiation, the initial wave of newly generated splenic B cells in self-reconstituting adults express a similar phenotype. In keeping with previous data on neonatal B cells, HSAhi cells from either normal or self-reconstituting mice are refractory to stimulation with anti-IgM antibodies, yet proliferate upon LPS stimulation, and generate primary antibody responses if given appropriate T cell help. In contrast to neonatal cells, HSAhi adult B cells are refractory to stimulation with PMA plus ionomycin. Together, these data suggest that peripheral HSAhi B cells in adults correspond to recently generated B cells, whose signaling characteristics are distinct from previously described B cell subsets.