A study of the antigenic potency of influenza virus inactivated by ultraviolet radiation has been made. Virus so inactivated is still capable of functioning as an immunizing agent when given to mice by the intraperitoneal route.

In high concentrations inactivated virus appears to be nearly as effective as active virus but when quantitative comparisons of the immunity induced by different dilutions are made, it is seen that a hundredfold loss in immunizing capacity occurs during inactivation.

Virus in suspensions prepared from the lungs of infected mice is inactivated more rapidly than virus in tissue culture medium.

A standard for the comparison of vaccines of epidemic influenza virus is proposed.

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