In chronic viral type B hepatitis, the presence in the serum of pre-S proteins of hepatitis B virus (HBV) envelope reflects viral replication. As peripheral blood mononuclear cells (PBMC) are known to be target cells for HBV replication, the aim of our study was to investigate the clinical relevance of pre-S protein expression in PBMC. Fifty-seven patients with chronic type B hepatitis and HBs antigenemia were studied. Following separation using the Ficoll gradient, the PBMC were lysed and studied for pre-S proteins by Western blot. HBs Ag and HBc/e Ag were assayed in parallel by radioimmunoassay. HBs Ag was detected in PBMC in 86 percent of cases, HBc/e Ag in 28 percent of cases and pre-S proteins in 34 percent of cases. A statistically significant association was found between the presence of HBc/e Ag in PBMC and both the serum HBe Ag (chi 2 test, p less than 0.01) and the serum viral DNA/DNA polymerase (t test, p = 2.10(-4)). The pre-S protein expression in PBMC was significantly associated with higher levels of DNA/DNA polymerase activity (chi 2 test, p less than 0.05). The expression of pre-S proteins in PBMC appears therefore to correlate with the HBV viral replication phase. The HBc/e Ag and pre-S protein detection in PBMC therefore offers a reliable non invasive approach to tissular viral replication. The clinical relevance of pre-S testing in PBMC was illustrated by the study of 12 cases of chronic active hepatitis positive for anti-HBe but with no or low level of serum DNA polymerase activity.(ABSTRACT TRUNCATED AT 250 WORDS)