Studies with knockout mice have indicated that the only isoform of phosphoinositide 3-kinase (PI3K) functioning in the oxidative burst of mouse neutrophils in response to heterotrimeric guanine nucleotide-binding protein-coupled receptor (GPCR) agonists is a class-IB PI3K, p110γ. In the present study, we observed that the cells from p110γ(-/-) mice gain a response to N-formyl-Met-Leu-Phe (fMLP) after priming with cytochalasin E. Even the unprimed cells, which show no response to fMLP, produce a significant amount of superoxide, when an effective agonist of the mouse-type fMLP receptors, Trp-Lys-Tyr-Met-Val-D-Met, is used to stimulate the cells. These results suggested that the class-IA isoforms (p110α, p110β, and p110δ) of PI3K are sufficient to trigger and maintain superoxide production. Examination of the effects of isoform-specific inhibitors suggested that the p110β isoform is the primary PI3K triggering the response to GPCR agonists when p110γ is absent.