Though oral microgranisms were among the first to be observed by humans, the interest in oral microbiology lagged. When it became apparent that the oral microflora did influence systemic disease of the body, interest was aroused in the nature and kinds of the microgranisms. The risk of infection in dental procedures is due to the abundant flora of the mouth. This hazard can be reduced to some extent by the use of a local disinfectant. The present studies were undertaken to evaluate and compare the various disinfectants which are commonly used in clinics and hospitals. The results were as follows. 1. The bactericidal activity of the disinfectants mainly depends upon the kinds of the agents, not upon the kinds of the microorganisms. 2. In H₂O₂(3%), the bactericidal activity was greatly related to the contact time. So, at least 4 minitues of contract time was required to use it as an oral antiseptic. 3. In ethyl alcohol (70%), Pseudomonas aeruginosa and Streptococcus salivarius surived a little after 15 seconds of contact time, but, no other colony was discovered after more than 15 seconds of contact time in any kins of microorganisms. 4. Merthiolate (0.1%) showed low antibacterial activity, more in Gram-positive organisms and less in Gram-negative organisms. 5. Benzalkonium chloride (0.1%) and povidone-iodine (10%) showed the most excellent results, revealing no surviving organisms only after 14 seconds of contact time.