Plasma samples from 89 asymptomatic hepatitis B surface antigen (HBsAg) positive volunteer blood donors were titrated for HBsAg by radioimmunoassay using the parallel line method. HBsAg titers ranged widely from 0.01 to 325 micrograms/ml. The titers correlated well with hepatitis B viral DNA (HBV DNA) and hepatitis B e antigen (HBeAg) in the serum. The HBsAg titers in 55 HBV DNA positive carriers were 90.7 +/- 61.7 micrograms/ml (Mean +/- SD) vs. 6.3 +/- 12.2 micrograms/ml in the 34 carriers without HBV DNA in the serum. The titers were 93.9 +/- 59.1 micrograms/ml in 56 carriers with HBeAg, 6.4 +/- 10.1 micrograms/ml in 24 anti-HBe-positive carriers, and 4.9 +/- 4.6 micrograms/ml in 9 HBeAg/anti-HBe-negative carriers. 50 (89.3%) of the 56 HBeAg-positive carriers had HBV DNA, in contrast to four (16.7%) of 24 anti-HBe-positive carriers. The study indicated that high-titered HBsAg carriers were much more likely to be infectious, and confirmed that HBeAg is a practical marker of infectivity. Absence of HBeAg, however, did not exclude infectivity in asymptomatic HBsAg carriers, inasmuch as one-sixth of the carriers had HBV DNA.