Intraperitoneal Millipore diffusion chambers were used to determine in vivo the fate of 7 strains of gram-negative bacilli when exposed to humoral defense mechanisms, yet protected from cellular elements. Such chambers also provided a means of collecting peritoneal fluid for bactericidal studies done in vitro. In contrast to findings in the guinea pig, all 7 strains grew in diffusion chambers in the mouse. Mouse peritoneal fluid failed to show in vitro any bactericidal activity. These results tend to support the belief that the mouse lacks a complement-dependent bactericidal system.