Counterimmunoelectrophoresis was used for the detection of ovalbumin in influenza vaccine preparations throughout a purification process applied to allantoic fluids of embryonated eggs. With an antiserum able to detect ovalbumin to a concentration of 0.31 microgram/ml, the progressive elimination of this contaminant was followed until the final preparation of purified concentrated vaccine. This easy procedure appeared to be specific and highly sensitive.