FcγRIIB functions as an inhibitory receptor to dampen B cell Ag receptor signals and immune responses. Accumulating evidence indicates that ex vivo B cells require the inositol 5-phosphatase, Src homology domain 2-containing inositol 5-phosphatase (SHIP), for FcγRIIB-mediated inhibitory signaling. However, we report here that LPS-activated primary B cells do not require SHIP and thus differ from resting B cells. SHIP-deficient B cell blasts display efficient FcγRIIB-dependent inhibition of calcium mobilization as well as Akt and extracellular signal-related protein kinase phosphorylation. Surprisingly, FcγRIIB-dependent degradation of phosphatidylinositol 3, 4, 5-trisphosphate and conversion into phosphatidylinositol 3, 4-bisphosphate occur in SHIP-deficient B cell blasts, demonstrating the function of an additional inositol 5-phosphatase. Further analysis reveals that while resting cells express only SHIP, B cell blasts also express the recently described inositol 5-phosphatase, SHIP-2. Finally, data suggest that both SHIP-2 and SHIP can mediate downstream biologic consequences of FcγRIIB signaling, including inhibition of the proliferative response.