Abstract The behavior of leukocytes, platelets, and erythrocytes was studied in the small vessels of rabbit ear chambers during the period of rapid clearance of bacteria subsequent upon the intravenous infusion of (1) a suspension of Streptococcus sanguis in the nonimmune animal and (2) a suspension of Diplococcus pneumoniae, Type III, in both immune and nonimmune animals. Immunity to the pneumococcus was demonstrated by a semiquantitative application of the capsular “swelling” reaction. Within 2 minutes the infusion of S. sanguis caused the sticking of all leukocytes to the endothelium and a concomitant reduction in and deformation of circulating platelets. Within 4 minutes platelets had aggregated and adhered to leukocytes. Erythrocytes exhibited an increase in crenated forms and stickiness to one another and to leukocytes and microthrombi. Flow diminished. After an hour leukocytes and platelets were circulating again and during the second hour were numerically on the increase while thrombi were enlarging and becoming hyaline. At 3–4 hours, rolling leukocytes were numerous, many normal-shaped platelets circulated, red cell aggregation and crenation were reduced, and hyaline thrombi were evident. Infusion of pneumococci in the immunized rabbit evoked a response essentially similar to that of S. sanguis except that red cell aggregation and crenation were more pronounced. In nonimmunized animals the response though milder was again similar but leukocytes and platelets continued to circulate although in diminished numbers, and during the final period of observation thrombi were insignificant. Thus in the main the response to bacteremia was analogous with that of particulate matter, macromolecular compounds, lipid emulsions, and dietary fat.