High antibody titres against rubella and measles viruses have recently been associated with certain chronic disorders such as chronic active hepatitis and occasionally with acute hepatitis. The present study was designed to evaluate the frequency of this association with acute hepatitis. Haemagglutination inhibition (HI) and complement fixation (CF) antibodies to rubella, measles and influenza A viruses were determined in 97 consecutive patients with acute viral hepatitis and in 97 control persons matched by sex and age. In patients, particularly in those over 30 years of age, a small but statistically significant increase in rubella HI titres was seen. In 3 hepatitis cases the rubella HI titres were very high (greater than or equal to 2 560). The antibody levels against measles and influenza A viruses did not differ from the controls. The present data indicate that a small proportion of acute hepatitis patients show a transient strong humoral immune response to rubella virus. The reason for this is not known.