A highly sensitive polymerase chain reaction (PCR) was used to analyse serum samples from HBsAg-positive patients in Sweden. Forty-two chronic carriers were tested, five of whom were of Swedish origin. Of the total, there were 13 HBeAg-positive and 27 anti-HBe-positive patients, while 1 patient was negative for both HBeAg and anti-HBe and one was positive for both markers. Nine of the 13 HBeAg-positive carriers and only 7 of the 27 anti-HBe-positive carriers had elevated alanine transaminase (ALT) levels (P = 0.01). Two PCR tests of marginally different sensitivity were used on all patient samples. All 13 HBeAg-positive patients and the patients with and lacking both HBeAg and anti-HBe markers, respectively, were positive in both PCR tests. One HBeAg-positive patient was shown to shed hepatitis B virus (HBV) DNA in both saliva and urine. Twelve of the 27 anti-HBe-positive carriers, 6 of whom had elevated ALT levels, were PCR positive. The remaining 15 had no evidence of HBV DNA and all but 1 had normal ALT levels. A positive PCR result was more common in those anti-HBe-positive patients with elevated ALT levels (P < 0.02). The precore gene from 18 samples was sequenced and, with a few exceptions, showed a high degree of conservation. We suggest that in the absence of optimal tests for infectivity of serum from HBsAg-positive patients, and until PCR becomes more widely available, all anti-HBe-positive patients with elevated ALT levels be considered highly infectious.