Summary: The data presented have shown that the rate of inactivation of the PR8 strain of influenza virus, Type A, by sodium oleate is rapid or gradual depending upon the ratio of the two reactants. For effective inactivation, the minimal ratio is, as previously reported, about 0.001M/per cent (oleate concentration/virus suspension). Sodium oleate at pH 7.5 has been shown to inactivate other strains of influenza virus in addition to PR8, including the Lee strain of influenza virus, Type B; WS (both neurotropic and pneumotropic activities); and the Melbourne and PR8 strains from chick embryo tissue culture. Of the new detergents tested upon the PR8 strain of virus, Duponol LS, Zephiran, Aerosols OT, MA, AY, IB, and OS, only the first three rendered the virus non-infectious. Immunization of mice with PR8 virus, which after inactivation with sodium oleate had been kept one day, one week, and one month in the ice-box, revealed that even after one month the virus was as effective antigenically as immediately after inactivation by soap.