Cultured macrophages exhibit spreading in response to external stimuli. It is relevant to in vivo morphologic changes of macrophages during extravasation, migration, and differentiation. The present study was performed to elucidate molecular mechanisms that regulate spreading of macrophages. Redox is a crucial factor that modulates a wide range of cell function. We found that macrophages undergo spreading in response to oxidant stress caused by hydrogen peroxide or an oxidant generating agent menadione. To identify signaling pathways involved, a role of mitogen-activated protein (MAP) kinases was investigated. Western blot analysis showed that treatment of macrophages with menadione rapidly induced phosphorylation of extracellular signal-regulated kinases (ERK1, ERK2) and p38 MAP kinase, but not c-Jun N-terminal kinase (JNK). Pharmacologic inhibition of either ERK or p38 activation blunted the macrophage spreading. Similarly, transfection with dominant-negative mutants of ERKs or a mutant p38 significantly suppressed the oxidant-triggered spreading. ERKs and p38 are known to activate serum response element (SRE) via phosphorylation of the ternary complex factor Elk-1. To further identify downstream events, we focused on a role of SRE. Stimulation of macrophages with menadione induced activation of SRE. Intervention in the SRE activation by a dominant-negative mutant of Elk-1 inhibited the menadione-induced spreading. These results suggest that oxygen radical metabolites, the well-known mediators for tissue injury, incite spreading of macrophages via the MAP kinase-SRE signaling pathways.