Abstract We have previously generated p85α−/− mice deficient for the most abundantly and ubiquitously expressed regulatory subunit of class IA PI3Ks. In p85α−/− mice, B cell development from pro-B cells to pre-B cells in the bone marrow is impaired and the number of mature B cells in the periphery is decreased. Here we further investigated the role of PI3K in early B cell development. First, we crossed p85α−/− mice with a transgenic mouse line expressing rearranged IgH and IgL chains Ig(H+L) specific for hen egg lysozyme (Hel-Tg). Introduction of functional Ig(H+L) chains did not restore the number of peripheral B cells in mice on a p85α−/− background. We next crossed p85α−/− mice with Rag-2−/− mice to generate p85α−/−Rag-2−/− double deficient mice. We compared the transition from B200+c-kit+CD25− pro-B cell stage to the B200+c-kit−CD25+ pre-B cell stage upon anti-Igβ antibody injection between Rag-2−/− mice and p85α−/−Rag-2−/− double deficient mice. While B200+c-kit−CD25+ cells were readily observed in Rag-2−/− mice after administration of anti-Igβ antibody, such transition was impaired in p85α−/−Rag-2−/− double deficient mice, indicating that the class IA PI3K is involved in the pre-BCR signaling. Our results indicate that class IA PI3K is important in pre-B cell antigen receptor-mediated signaling to induce pre-B cell expansion rather than the Ig gene rearrangement.