Summary: The intraperitoneal inoculation of Gramnegative bacteria into mice was followed by morphologic changes in these organisms from typical rods to protoplasts to protoplasmic membranes. The percentage of protoplast formation was a function of challenge size in both normal mice and those with elevated nonspecific resistance. Low challenges resulted in an acceleration of this effect upon cells, whereas in mice possessing specific antibody, the transformation of cells was rapid with maximum nonlethal doses. An additive protective effect was obtained in mice possessing both specific immunity and elevated nonspecific resistance. These mice were capable of transforming large challenge inoculums into protoplasts. Evidence also has been presented indicating an active mouse complement system which may be demonstrated in freshly drawn blood.