One hundred and forty-eight sera from 101 cases of acute postnatal rubella were examined by the radial haemolysis (RH) technique currently recommended by the Public Health Laboratory Service of the United Kingdom. In 75 cases the date of onset of rash was known. No sera were RH positive until the third day after the onset of rash and one serum was still negative at 9 days. By 10 days all sera gave a zone of at least 10 mm. The antibody response detected by RH appears later than that detected by haemagglutination inhibition (HI). Thirty-nine of the 101 cases could be diagnosed by seroconversion or a significant rise in titre using the HI test (diagnosis of the remainder depending upon rubella-specific IgM tests). Using the RH test this figure rises to 48 but in ten of these cases reliance on RH would have meant a delay in diagnosis. The results also indicate that many more sera would need to be tested for rubella-specific IgM if the RH test was used instead of the HI test for evaluating possible cases and contacts of rubella.