Phosphoinositide kinase (PI3K) is activated by various receptors on lymphocytes and regulates development, activation, and tolerance. Genetic ablation of PI3K function in T cells leads to the appearance of autoimmune disorders. In B cells, loss of the class IA regulatory subunit p85alpha causes a partial defect in B cell development and proliferation, whereas loss of p85beta alone causes no apparent changes in B cell function. Here we investigate further the consequences of p85beta deletion in B cells, in the presence or absence of p85alpha. We demonstrate that p85beta partially compensates for loss of p85alpha in B cell development and peripheral survival, with greater defects observed when both isoforms are absent. BCR-mediated AKT phosphorylation is partially reduced in p85alpha-deficient B cells and further diminished with concomitant loss of p85beta. Unexpectedly, loss of p85beta results in increased BCR-mediated proliferation and ERK phosphorylation. These results indicate that the p85beta regulatory isoform has partially overlapping functions with p85alpha in B cells as well as a unique role in opposing BCR responses.