Comparative studies between amoxycillin (AMPC) and penicillin G (PCG) were carried out in order to clarify whether AMPC can take place of PCG in the chemotherapy of subacute bacterial endocarditis (SBE). The results were as follows : 1) The minimal inhibitory concentration (MIC) of AMPC against Streptococcus mitis isolated was 0.2 μg/ml. Serum levels of AMPC from 30 minutes until 6 hours after oral administration to 5 healthy adult volunteers at the dose of 1 ranged from 1. 0 μg/ml to 7. 8 μg/ml on the average, which were 5 to 40 times higher than MIC of AMPC against S. mitis. 2) The serum bactericidal activities after oral administration of AMPC at the dose of 1 to a patient with SBE were compared with those of PCG after intramuscular injection at the dose of 400, 000 or 600, 000 units. Reciprocal titer of bactericidal activities of AMPC against S. mitis was 4 to 60 times and it was higher than those titers after 400, 000 units PCG injection, being almost same as those of 600, 000 units injection. From the above mentioned results, it is considered to be useful to give AMPC orally at the dose of 1 g 6 hourly in a day to patients with SBE instead of PCG injection.