Because of the technical problems and length of time involved in the satisfactory performance of the haemagglutination-inhibition test for detection of rubella-specific antibodies, a commercially available passive haemagglutination (PHA) kit utilising rubella antigen-sensitised human erythrocytes was tested for its suitability for use in diagnostic laboratories. The immune response to acute rubella infections as measured by PHA was considerably delayed compared to the response measured by haemagglutination-inhitition. Titres of rubella-specific antibody only became comparable six months after the infection. The commercially available PHA kits is a useful addition to diagnostic laboratories for the determination of immune status and, in conjunction with the haemagglutination-inhibition test, can be an indicator of recent rubella infection.