We examined the effect of bacterial wall lipopolysaccharide (LPS), a strong inflammatory agent, on the morphology, cell motility, cytoskeletal organization, and phagocytic activity of microglia in tissue cultures initiated from neopallia of newborn C3H/OuJ mice. Normally, the microglia in our cultures are non-migratory and Mac-1 positive, have ameboid cell morphology, no polarity, many short processes that extend into lamellipodia in opposing directions, and undulating cell membrane projections. When 1-5 micrograms/ml LPS is added to such cultures, some cells acquire polarity by forming a large lamellipodium and begin to migrate. Two hours later migration ceases; the membrane undulations stop; and the cells become non-polar, assume a large, round, flat shape, and gradually develop many microspikes all over the cell body. Those cells that do not transform into large, round, flat cells enlarge and extend numerous lamellipodia in opposing directions. We found that the cytoskeleton of microglia is composed of actin, vimentin-containing intermediate filaments (IF) and microtubules (MT). Vimentin-containing IF and MT form dense networks that radiate into the cell periphery, whereas F-actin is diffusely arranged throughout the cytoplasm. The LPS-treated cells show changes in the organization of the main components of the cytoskeleton. F-actin is reorganized by the formation of bundles underneath and parallel to the cell membrane and other bundles projecting into the cores of the microspikes. The vimentin-containing IF dense network reorganizes into two condensed rings, with fine strands of IF extended between the two rings and the MT networks become less dense and extend throughout the cytoplasm. The LPS treatment potentiates the phagocytic activity of the microglia. However, approximately 30% of microglia lose the expression of MHC class II antigens.