Since previous studies did not provide conclusive data regarding the presence of hepatitis B surface antigen (HBsAg) in the feces of HBsAg carriers, the feces of 20 HBsAg carriers and six patients with HBsAg-positive active liver disease were examined with use of a reproducible method for concentrating and detecting HBsAg in feces in which bovine serum, which has been shown to protect HBsAg destruction by fecal components, was added to the fecal specimens. HBsAg was not detected in 66 fecal specimens from the 26 HBs-Ag-positive persons. Furthermore, HBsAg could not be detected in the feces of a carrier who had ingested 4 ml of his own serum. This finding suggests that there are factors in the gastrointestinal tract that interfere with the immunoreactivity of the HBsAg and possibly destroy the hepatitis B virus. These findings explain why oral/fecal spread does not play a major role in the transmission of hepatitis B virus.