In the previous communication, the mode of action of streptothricin-3 on the growth of influenza virus has been studied in a tissue culture system using chorioallantoic membrane as host cells. In the studies to be reported here, chartreusin, chemically quite different antivirals from streptothricin-3 has been tested just in the same manner as was done in a previous examination.<br>The isolation of crystalline chartreusin from a filtrate of a streptomyces designated as G72, and the mode of action of the agent on the growth of T₃ phage has been already reported by Kaneko.<br>The result of this study will be summarized as follows:<br>1) The potency of the substance did not destroyed when incubated at 37°C for 24 hrs., in the tissue culture system employed.<br>2) As low as 25mcg/ml was the minimum inhibitory concentration when 10⁵TCID₅₀ of the PR₈ virus was used as inoculum. However, with concentration of 12.5mcg/ml, comparable good growth to the control was obtained. In other words, changing small range of concentrations gave considerable difference on the inhibition rate. This fact was also observed by Anzai, independently.<br>3) Sensitivity phase during the course of virus development to this agent was examined and the addition of the substance 3 hrs. after infection did not inhibit the virus growth, while the addition at 0 and 1 hr. gave complete inhibition. This finding will suggest that chartreusin sensitive stage may exist earlier than that of streptothricin-3.<br>4) Depending upon the inoculum size, the effect of the substance on the growth of virus varied. This fact, however, has to be studied more throughly in a light of Anzais work, in which he found the contrary result by different analytical procedures.<br>5) Reversibility of the action of the substance to the host cell was observed when normal chorioallantoic membrance was treated with the substace for 5 hrs. before infection.<br>All of the evidence illusrated here may suggest the different site of action of the afore-mentioned 2 substances, i.e. streptothricin-3 and chartreusin and will be discussed further in detail in an accompanying paper.