A simple single-radial-haemolysis technique is described that permits the detection and assay of antibody to influenza virus haemagglutinin. The method depends on the passive haemolysis of virus-treated erythrocytes by antihaemagglutinin antibody and complement. Under the "standard" test conditions described antibody to the other surface antigen of the influenza virus (neuraminidase) or to the internal antigens of the virus (nucleoprotein and matrix protein) do not produce haemolysis. Because it requires only small amounts of crude virus antigen and is rapid and simple, the method appears to be of considerable value for large-scale seroepidemiologic studies of new influenza virus variants. Antihaemagglutinin antibody detected by single-radial-haemolysis appears to be relatively strain-specific; the technique may therefore be useful in the antigenic characterization of virus isolates.