In the time course of the infectious process, 239 patients with acute hepatitis B, 220 patients with chronic hepatic diseases with HBs-antigenemia and carriers of HBs-Ag were tested for the presence of HBe-Ag were tested for the presence of HBe-Ag and antibody to it using double immunodiffusion test (DID) and radioimmunoassay (RIA) or enzyme-labeled immunosorbent assay (ELISA). At high HBs-Ag concentrations determined by DID the HBe antigen was found significantly more frequently than at low concentrations detectable by the sensitive tests only, whereas antibodies to HBe were detected more frequently at low concentrations of HBs-Ag. In patients with acute hepatitis B examined in the first week after the onset HBe-antigen was found in 85.8% cases (38.9% by DID and 46.9% by RIA or ELISA alone). The HBe-antigen could be detected for over 2 months after the onset in 3.6% of the patients in whom the pathological process was becoming chronic. A correlation between the e-system detection and the clinical course of the disease was established: in severe forms, e-antigen concentrations detectable by DID were observed significantly more frequently than in mild (16.7%) or moderately severe (29.2%) forms, and at the same time e-antibody was found more frequently (28.6% against 12.5 and 10.4%, respectively) early in the acute period. The early appearance of HBe-antibody in fulminant course of acute hepatitis B was typical mainly in babies of the first 6 months of age (75%) and in the majority of the cases was indicative of immediate unfavourable prognosis.