The mammalian Ras GTPase-activating protein (p120Ras-GAP) interacts with activated members of the Ras superfamily of GTP-binding proteins to accelerate their deactivation by sharply increasing their rates of GTP hydrolysis. Among the Ras-family proteins interacting with p120Ras-GAP is Rap1A/Krev1, whose activity is not affected by p120Ras-GAP but which competes with Ras for p120Ras-GAP. A second protein that interacts with p120Ras-GAP is P190Rac-GAP, which activates the GTPase of guanine nucleotide-binding proteins of the Rho family (including Rac1 and Rac2). Both these p120Ras-GAP-binding proteins are of interest in connection with the regulation of the respiratory burst oxidase, Rap1A/Krev1 because it copurifies with cytochrome b558 and p190Ras-GAP because it inhibits the Rac2-dependent activation of the respiratory burst oxidase in a cell-free system. Using an 18-mer antisense oligonucleotide, we were able to decrease the expression of p120Ras-GAP in Epstein-Barr virus-transformed B lymphocytes. Under conditions where p120Ras-GAP expression was significantly depressed by antisense oligonucleotides, we observed a 40% increase in protein kinase C-dependent but not receptor-dependent O2 production. In contrast, sense and scrambled oligonucleotides had no effect on either p120Ras-GAP expression or O2 production. Our results suggest a role for p120Ras-GAP as a negative regulator in the protein kinase C-mediated activation of the respiratory burst oxidase.