The use of gamma radiation from a cobalt-60 cell for the preparation of non-infective diagnostic antigens for influenza A, influenza B, mumps, smallpox, and herpes simplex has been investigated. It was found possible to destroy the infectivity while retaining most of the complement-fixing activity of all these antigens. The degree of purity of the antigen had no apparent effect on the rate of inactivation, as is the case when formaldehyde is used. Under the experimental conditions described, the degree of inactivation depended on the total amount of radiation applied and not on the dose rate. The kinetics of virus inactivation make it possible to calculate the amount of radiation required to destroy infectivity completely and yet retain most of the antigenicity. If necessary it is possible to apply an additional calculated amount of radiation to destroy residual infectivity without causing loss of antigenicity. Gamma radiation appears to be superior to formaldehyde treatment for the preparation of the herpes simplex antigen which is particularly sensitive to heat and to formaldehyde.