The use of salivary samples to diagnose acute viral hepatitis was investigated. Tests for IgM antibody to hepatitis A virus (anti-HAV) on 29 acute-phase samples from serologically confirmed cases of hepatitis A were strongly reactive. Follow-up samples indicated that IgM anti-HAV persisted at moderate levels for 2-4 months and was not usually detectable thereafter. The ratio of IgM to IgG anti-HAV (RIA index) correlated closely with the interval from onset of infection. Significant levels of IgM anti-HAV were not detected in the saliva of 103 IgG anti-HAV positive and 102 IgG anti-HAV negative individuals nor of 4 individuals with hepatitis B. Similarly, IgM anti-HBc was present in the saliva of acute cases of hepatitis B, but not in the saliva of 25 IgG anti-HBc positive and 85 IgG anti-HBc negative individuals, nor of 24 individuals with recent hepatitis A. It is concluded that saliva is a convenient and satisfactory alternative to serum for the diagnosis of hepatitis A infection.