AbstractMicroglia‐derived superoxide is critical for the inflammation‐induced selective loss of dopaminergic (DA) neurons, but the underlying mechanisms of microglial activation remain poorly defined. Using neuron‐glia and microglia‐enriched cultures from mice deficient in the MAC1 receptor (MAC1−/−), we demonstrate that lipopolysaccharide (LPS) treatment results in lower TNFα response, attenuated loss of DA neurons, and absence of extracellular superoxide production in MAC1−/−cultures. Microglia accumulated fluorescently labeled LPS in punctate compartments associated with the plasma membrane, intracellular vesicles, and the Golgi apparatus. Cytochalasin D (CD), an inhibitor of phagocytosis, blocked LPS internalization. However, microglia derived from Toll‐like receptor 4 deficient mice and MAC1−/−mice failed to show a significant decrease in intracellular accumulation of labeled LPS, when compared with controls. Pretreatment with the scavenger receptor inhibitor, fucoidan, inhibited 79% of LPS accumulation in microglia without affecting superoxide, indicating that LPS internalization and superoxide production are mediated by separate phagocytosis receptors. Together, these data demonstrate that MAC1 is essential for LPS‐induced superoxide from microglia, implicating MAC1 as a critical trigger of microglial‐derived oxidative stress during inflammation‐mediated neurodegeneration. © 2007 Wiley‐Liss, Inc.