Two hundred and twelve saliva specimens of HB-SAg carriers were tested for HB-SAg by crossover immunoelectrophoresis (CIEP) and radioimmunoassay (RIA) after concentration, between April 1971 and September 1973. They were divided into 3 groups (A, B, and C). CIEP detected HG-SAg in 6 per cent in Group A (66 specimens) and in 12 percent in Group B (87 specimens), and in 78 per cent in Group C. Different methods of preparation and concentration of saliva specimens were discussed. The positive results by RIA were confirmed by a blocking test with rabbit HB-SAg antiserum. Intermittent presence of HB-SAg in repeated saliva specimens from individual carriers was found. Infected saliva of HB-SAg carriers may be of importance in transmission of hepatitis B infection.