We studied the interaction between HBV and HDV infection in 149 consecutive subjects with HBsAg positive chronic hepatitis and in 22 chronic HBsAg healthy carriers. Liver HBcAg was detected in 52 (30.4%) of the 171 subjects. Of these 52, 35 were HBV-DNA and HBeAg positive, 11 HBV-DNA positive only; two HBeAg positive only and four were negative for both HBeAg and HBV-DNA. None of the 119 HBcAg-negative subjects had detectable HBV-DNA in serum. HD-Ag in hepatocytes was detected in 31 of the 171 subjects (18%); it was detectable in none of the 22 HBsAg healthy carriers, in four of the 56 patients with chronic persistent hepatitis (7.2%), in six of the 24 patients with chronic lobular hepatitis (25%), in 16 of the 40 patients with chronic active hepatitis (40%) and in five of the 29 with cirrhosis (17%). A presence of anti-HD in serum in the absence of liver HD-Ag was found in 54 of the 171 subjects (32%). This condition was observed not only in patients with a progressive disease (37.7% of chronic active hepatitis or cirrhosis and 33% of chronic lobular hepatitis), but also in healthy carriers (36%) and in chronic persistent hepatitis patients (21.4%). Liver HBcAg was detected in 6.4% of the 31 HD-Ag-positive patients, in 12.9% of the 54 HD-Ag-negative/anti-HD positive, but in 50% of the 86 with no marker of HDV infection. HDV appears to inhibit HBV genome and such inhibition may persist even when anti-HD is the only HDV marker detectable.