Observations are presented on the course of experimental bacteriemia in rabbits modified by thorotrast reticuloendothelial "blockade" and mechlorethamine induced leukopenia, given singly and together. The production of severe granulocytopenia did not impair the clearance of staphylococci from the blood stream and did not enhance the impairment of clearance produced by thorotrast reticuloendothelial "blockade." Neither severe granulocytopenia nor thorotrast blockade alone produced consistent impairment in the removal of circulating E. coli from the blood stream, whereas when these conditions were present together, the early phases of clearance in these animals were virtually prevented. These results indicate that the host mechanisms operative in the control of bacteriemia differ for staphylococci and E. coli. It is suggested that the fate of bacteria within polymorphonuclear leukocytes may in part determine the importance of these cells in the removal of bacteria from the blood stream.