Phosphatidylinositol (PI)-3 kinase has been implicated in several aspects of intracellular membrane trafficking, although the detailed mechanism is yet to be established. We previously reported that wortmannin (WT), a selective inhibitor of PI-3 kinase, inhibited the bone-resorbing activity of osteoclasts (Nakamura et al., 1995, FEBS Lett., 361:79-84). In this study, we examined how PI-3 kinase was involved in membrane trafficking in osteoclasts which are primary bone-resorbing cells. Osteoclasts exhibit a highly polarized cytoplasmic organization, the ruffled border. Ruffled borders are formed by numerous deep membrane invaginations, on which vacuolar H(+)-ATPase (V-ATPase) is localized in a high density. Immunoelectron microscopic analyses revealed that PI-3 kinase was specifically present along ruffled border membranes and the limiting membranes of associated intracellular vacuoles in rat authentic osteoclasts. WT and LY294002, another inhibitor of PI-3 kinase, caused the accumulation of numerous acidic vacuoles which were stained with acridine orange in murine osteoclast-like multinucleated cells formed in vitro. An electron microscopic examination showed that these vacuoles contained V-ATPase along their limiting membranes and appeared to be derived from the Golgi apparatus as ruffled border precursors. A time course study revealed that WT-induced vacuoles began to accumulate in the region close to the apical membrane and were finally distributed throughout the cytoplasm. Removal of WT from the culture medium resulted in the disappearance of vacuoles in the cytoplasm, leading to the formation of ruffled borders. During the culture period, some vacuoles were observed to fuse with the ruffled border membrane. A pit formation assay on dentine slices also showed that the pit-forming activity of osteoclast-like cells was recovered by the removal of WT from the assay. These results suggest that PI-3 kinase plays an important role in ruffled border formation in osteoclasts, probably in the fusion of membrane vacuoles with the plasma membrane.