To escape from reactive oxygen species generated in response to infection, intracellular pathogens, such as Salmonellae and Anaplasma phagocytophilum, can modulate the expression and distribution of the phagocyte NADPH oxidase components. In the present study, we analyzed the effect of Ehrlichia chaffeensis infection on the ability of human monocytic THP-1 cells to produce superoxide anion. Phorbol ester-stimulated superoxide generation and mRNAs for gp91^phox, p47^phox, and p67^phox were significantly increased in E. chaffeensis-infected THP-1 cells. These increases were also achieved in THP-1 cells cultured in medium conditioned by the E. chaffeensis-infected cells, indicating that bystander cells can be activated for superoxide generation and implicating soluble factors in the response. Heat-stable and -unstable factors represented one-third and two-thirds of this activity, respectively. These results suggest that immature human monocytic cells increase their ability to generate superoxide anion in response to E. chaffeensis infection regardless they have been directly invaded or not.