AbstractWe have investigated the effect of therapeutic doses of diazepam (7μg/mouse) on the association of actin with the macrophage cytoskeleton using cytochemical and morphological methods.Results obtained indicated that diazepam was able to modulate the content of actin in macrophages; such an effect proved to be time-dependent. After fixation and staining for indirect immunofluorescence with actin antibody, peritoneal macrophages from mice treated for short time with diazepam, showed a fluorescent intensity increase compared to control mice. The fluorescent intensity augmented reaching peak value within 14 days of treatment. Afterwards, this value dropped below control value for mice that underwent longer treatments. In thein vitroexperiments concentrations of 10−5M, diazepam inhibited a well cell spread and a lower amount of actin after 15 min of incubation was also revealed.These results suggest that administration of diazepamin vivoplays a role in both the nonspecific and specific immune response, producing in the macrophages a reorganization process of microfilaments.