From 200 acute viral hepatitis patients serum samples were collected successively, at various intervals. In 192 patients one single hepatitis B-antigen (HB-Ag) subdeterminant ("ad" in 37.6% and "ay" in 62.4% of the cases) occurred and persisted during the entire course of the disease. The remained 8 patients (4%) showed an alternation of HB-Ag subdeterminants. The possible causes of this alternation are discussed in detail.