The effect of antibiotic prophylaxis on bacteraemia was assessed in 60 oral surgery operations. Fifty-one patients were divided in three groups receiving a 2 g dose of phenoxymethylpenicillin, a 0.5 g dose of erythromycin or no prophylaxis. During operation, the total numbers of bacteria were lower in the antibiotic groups than in the non-treatment groups (P less than 0.05). The number of patients with anaerobic bacteraemias, however, were similar in all three groups. The incidences of bacteraemia 10 min after operation were significantly lower in the antibiotic groups (P less than 0.05). The minimum inhibitory concentrations (MICs) of phenoxymethylpenicillin were below 2 mg/l in 93% of the strains and the minimum bactericidal concentrations (MBCs) were below this value in 80% of the strains. The MICs of erythromycin were below 2 mg/l in 80% of the isolates, and the MBCs were between 4-32 mg/l in 62% of the isolates.