Penicillin has a strong effect on hemolytic streptococcus, viridans streptococcus, pneumococcus, etc., while the sensitivity of enterococcus is relatively poor. Staphylococcus aureus and the majority of epidermidis that do not produce penicillinase are sensitive to penicillin, whereas Staphylococcus aureus that produces penicillinase is highly resistant to it.