Expression of a human C-reactive protein (CRP) transgene has been shown to protect mice from lethal Streptococcus pneumoniae infection. In the present study, we used cobra venom factor-induced decomplementatiun to investigate the role of complement in this CRP-mediated protection. An intact complement system significantly reduced pneumococcal bacteremia at 24 h postinfection and extended median survival time of both CRP-transgenic and nontransgenic mice. However, mortality was significantly lowered only for CRP transgenic mice. The transgene significantly reduced bacteremia for both normocomplementemic and decomplemented mice, but it resulted in a significantly longer median survival time and lower mortality only for normocomplementemic mice. These data suggest that in vivo complement and CRP amplify each other's protective capacity, particularly during the early course of infection.