The contributions of granulocytes to the prevention and therapy of Streptococcus sanguis endocarditis with procaine benzylpenicillin (PBP) was investigated in rabbits. Depletion of granulocytes by treatment with mechlorethamine appeared to have no significant effect on either the prophylactic or therapeutic activities of PBP. Administration of 3,000 IU of PBP before inoculation with S. sanguis retarded the course of the endocarditis for only 24 h whether granulocytes were normal or depressed in numbers. Prophylaxis with either 15,000 or 30,000 IU of PBP was equally effective in non-granulocytopenic and granulocytopenic rabbits. Treatment of established infections with PBP at doses of 3,000 to 300,000 IU of PBP at 12-h intervals for 48 h was equally effective in rabbits with normal and depressed numbers of granulocytes. The effect of 3,000 IU of PBP was equivalent, however, to that of granulocytes alone, as shown by the fact that the numbers of CFU per gram of vegetation in the granulocytopenic rabbits treated with this dose of PBP and in the non-PBP-treated control rabbits were not significantly different.