The effect of tyrosine phosphorylation of PI3K on its enzymatic activity is quite controversial, and the molecular mechanism by which ROS trigger PI3K membrane relocation is unclear. Therefore, we investigated the regulatory mechanism of hydrogen peroxide-induced PI3K activation in DT40 cells, utilizing genetic and pharmacological approaches. Our results revealed that hydrogen peroxide induced tyrosine phosphorylation of the p110 but not the p85 subunit of PI3K in DT40 cells. This phosphorylation was intact in Btk- and Cbl-deficient DT40 cells, but was drastically suppressed in Lyn, Syk, or BCAP-deficient DT40 cells. Tyrosine phosphorylation of p110 did not alter its catalytic activity, and hydrogen peroxide stimulation did not cause an increase in the intrinsic PI3K activity; however, hydrogen peroxide stimulation did induce PI(3,4,5)P3 accumulation and activate Akt. The activation of Akt, as monitored by its ability to phosphorylate GSK-3alpha/beta and by its S473 phosphorylation, was strictly dependent on PI3K activity. Under our conditions, hydrogen peroxide-induced PI3K and Akt activation was independent of Lyn, Syk, Cbl, BCAP, or Ras when each was eliminated individually either by mutation or by a specific inhibitor. In comparison, Akt activation by B cell receptor cross-linking was dependent on BCAP. In addition, hydrogen peroxide treatment caused an increase in the amount of p85 PI3K associated with the particulate fraction. Together, these results indicate that the hydrogen peroxide-induced PI3K and Akt activation in DT40 cells was achieved through PI3K membrane recruitment to its substrate site, thereby enabling PI3K to maximize its catalytic efficiency.