In this report we present data on the ability of murine eosinophils to generate inositol phosphate derivatives, and their relationship with the activation of 5'-lipoxygenase by a Fc-gamma R-dependent mechanism. The addition of anti-IgG F(ab')2 to mouse eosinophils, previously sensitized with IgG, induces inositol phosphate generation after 2 min and after 10 min of stimulation. Maximal generation of inositol tris and inositol tetrakis phosphate has been detected after 15 min of stimulation, and the optimal concentration of anti-IgG F(ab')2 was found to be 25 micrograms. Inositol tris phosphate formation is also observed at 5 min after the addition of the calcium ionophore A23187 (5 microM). We also report that neomycin, an inhibitor of phosphoinositide-phospholipase C, inhibits Fc-gamma R-mediated phosphoinositide breakdown in a dose-dependent manner (88% inhibition at 150 microM of neomycin). The possible involvement of phosphoinositide breakdown in the activation of 5'-lipoxygenase has been investigated. Using streptolysin-O permeabilized cells and different doses of neomycin that inhibit phosphoinositide breakdown, we have demonstrated a parallel decrease in LTC4 released by these cells, using either A23187 (86% inhibition at 200 microM of neomycin) or anti-IgG F(ab')2 (82.4% inhibition at 100 microM of neomycin). [Ca2+]i elevation has been observed by loading the cells with the fluorescent calcium indicator Fura-2 penta-acetoxy methyl ester and after stimulating with the anti-Fc-gamma RII mAb (2.4G2). It is likely that the activation of murine eosinophils by a Fc-gamma R mechanism stimulates phosphoinositide breakdown as a primary step that leads to the activation of murine 5'-lipoxygenase, producing the formation of leukotriene C4.