Paired serum and saliva specimens were tested by conventional assays and by IgG-capture radioimmunoassays (GACRIA) and enzyme-linked immunosorbent assays (GACELISA) for antibody to hepatitis A virus (HAV, 100 pairs), human immunodeficiency virus (HIV, 53), hepatitis B virus core (HBc, 62), and rubella virus (30). Conventional assays failed to detect viral antibodies in the saliva of 93 of 119 seropositive subjects. However, GACRIA detected the antibodies in both serum and saliva of all subjects seropositive by conventional tests, except 2 saliva specimens false-negative for anti-HBc and 1 false-negative for anti-rubella-virus. For anti-HIV and anti-HBc serum and saliva GACRIA reactivities did not differ significantly, but anti-HAV and anti-rubella-virus GACRIA reactivities were stronger in serum than saliva. GACRIA and GACELISA results on saliva for the four antibodies correlated closely; for anti-HAV and anti-HIV GACELISA and GACRIA were equally accurate. For both saliva and serum, GACRIA was superior to an IgA-capture assay in detecting anti-HAV and anti-HIV.