The major influenza A virus structural antigens, matrix protein, nucleoprotein, haemagglutinin and neuraminidase were measured rapidly and accurately using a rocket immunoelectrophoresis technique. Virus was disrupted with 1% (w/v) sodium sarcosyl and electrophoresed into agarose containing specific antiserum to the individual virus structural proteins in 0.05 M-barbitone buffer, pH 8.6, for 1 to 4 h. For haemagglutinin antigen assays statistical analysis indicated that the coefficient of variation within an immunoelectrophoresis plate was 8.0% for antigen concentrations in the range 15 to 40 microgram/ml protein. For haemagglutinin and matrix protein the method was sufficiently sensitive to measure concentrations of antigens as low as 1.5 and 2.0 microgram/ml respectively. By incorporation in the agarose of mixtures of antisera against specific antigens of the virus, haemagglutinin, matrix or nucleoprotein could be assayed simultaneously.