ABSTRACT—Serum specimens collected from patients with chronic type B hepatitis during their acute exacerbations were studied with radioimmunoassay for serological markers of delta (δ) agent infection. Serum δ antigen was negative in all 56 hepatitis B e antigen (HBeAg)‐positive and 25 anti‐HBe‐positive acute exacerbations. Two (3.6%) HBeAg‐positive and five (20%) anti‐HBe‐positive acute exacerbations were found to be positive for both IgG anti‐δ and IgM anti‐δ. The incidence in anti‐HBe‐positive patients was significantly higher than in HBeAg‐positive patients. These results indicated that some of the acute exacerbations, particularly in anti‐HBe‐positive phase, could be attributed to δ‐superinfection. However, acute exacerbations preceding HBeAg/anti‐HBe seroconversion were