Cellular locomotion results from a series of spatially and temporally integrated reactions. The coordinated regulation of these reactions requires sensitive intracellular signaling mechanisms. Because protein phosphorylation reactions represent important signaling mechanisms in mammalian cells, we investigated the effect of okadaic acid, a phosphoprotein phosphatase inhibitor, on protein phosphorylation and macrophage motility. Okadaic acid was applied to rat alveolar macrophages, and motility was quantitated by a directed chemotaxis assay. Okadaic acid inhibits macrophage motility in a dose-dependent fashion; the concentrations for 50 and 100% inhibition were 3 and 25 microM, respectively. Protein phosphorylation studies demonstrated a 2.5-fold increase in total protein phosphorylation in macrophages treated with 25 microM okadaic acid. These experiments also demonstrated a dose-dependent increase in the phosphorylation of the 20-kDa light chain of myosin. Moreover, 25 microM okadaic acid 1) maximally increased myosin light chain phosphorylation by 6.6-fold, 2) raised the level of myosin associated with the cytoskeleton from a basal level of 47.0 to 96.7% of the total myosin, and 3) induced profound morphological changes as visualized by scanning electron microscopy. These data correlate an increase in protein phosphorylation with a decrease in macrophage motility. Furthermore, they suggest that phosphoprotein phosphatase inhibition may prevent motility by uncoupling coordinated processes, such as cytoskeletal reorganization, that are essential for macrophage motility.