Abstract For 27 of the 50 patients who had not received an antibiotic, cultures of blood drawn immediately after oral surgical procedures yielded bacteria. Ten of these positive cultures grew streptococci of the mitis group, and 1 yielded Staphylococcus aureus . For 8 of a similar group of 50 patients who had received phenoxymethyl penicillin preoperatively every 6 hours for 4 doses totaling 4,000,000 U., cultures of blood drawn immediately after surgery grew bacteria; however, no streptococci of the mitis group were encountered. When half this dose, or 2,000,000 U., was used for 27 patients, cultures of blood immediately after surgery from 5 grew bacteria; the blood of one of these 5 yielded Streptococcus mitis . Phenoxymethyl penicillin is effective in decreasing the incidence of bacterial invasion of the blood after oral surgery. When given in doses employed in this study, the agent seems to minimize invasion of the blood by streptococci of the mitis group. A total of 4,000,000 U. given preoperatively in 4 equally divided doses proved as effective as have other forms of penicillin given on differing schedules by others.