We examined the prevalence of serum anti-delta antibody among 769 intravenous drug abusers in Taiwan. delta infection was found extremely common in the HBsAg-positive carriers with drug abuse, because 128 (85%) of 151 carriers were positive for anti-delta. However, most of antibody titers were low (less than 1:5,000). By molecular hybridization, delta RNA genomes were detectable in only five (4%) of 128 asymptomatic carriers positive for anti-delta. The results suggested that most of them had previous, instead of ongoing, delta infection. We also studied the serum markers reflecting hepatitis B virus replication in these carriers and a control group. Hepatitis B e antigen was positive in only 12 (10%) of 115 anti-delta positive carriers in contrast to 22 (23%) of 95 age- and sex-matched HBsAg-positive, anti-delta negative volunteers donating blood (p less than 0.025). Furthermore, in contrast to the close association between HBeAg and hepatitis B virus DNA generally present in HBsAg carriers (concordance in this series: 79%), among the 12 HBeAg-positive subjects of the delta infected group, only two, or 17%, had serum hepatitis B virus DNA (p less than 0.003). Such suppressing effects did not require a simultaneous presence of the delta agent, as shown by the lack of delta RNA genomes in the serum by sensitive assay. We conclude that although delta superinfection is common in the asymptomatic HBsAg carriers with intravenous drug abuse in the nonendemic area of Taiwan, continuous delta activities are uncommon in them. In addition, the previous delta infection probably exerts effective suppression on the hepatitis B virus replication in these HBsAg carriers.