FcgammaRI, the human high-affinity IgG receptor, is responsible for the internalization of immune complexes and their subsequent targetting to the lysosomes for degradation. We show here that aggregation of FcgammaRI by surface immune complexes in interferon-gamma-primed U937 cells causes the transient appearance of swollen vacuolar structures, probably swollen late endosomes, which disappear as the immune complexes are degraded. Wortmannin and LY294002, specific inhibitors of phosphoinositide 3-kinases (PI 3-kinases), delay the disappearance of these structures and also correspondingly inhibit degradation of FcgammaRI-mediated immune complexes. In addition these inhibitors delay the initial phase of FcgammaRI-mediated endocytosis of immune complexes and block the activity of FcgammaRI-stimulated phospholipase D, an enzyme that has previously been implicated in membrane-trafficking events. p85 is the regulatory subunit of PI 3-kinase. A p85-dependent PI 3-kinase was shown to be involved in the initial phase of FcgammaRI-mediated endocytosis, but not in the trafficking of immune complexes for degradation or the activation of phospholipase D. The results presented here show a role for a p85-independent PI 3-kinase in regulating the trafficking of FcgammaRI-mediated immune complexes, either directly or as a result of the activation of phospholipase D, and a distinct role for a p85-dependent PI 3-kinase isoform in the initial phases of FcgammaRI-mediated internalization of immune complexes.