Anti-T agglutinin was studied in normal healthy subjects and in cirrhotic patients before and after oral administration of antibiotics depressing intestinal microbial flora. After antibiotic treatment anti-T levels were significantly reduced. These observations show that bacterial antigens of intestinal origin are likely to be responsible for anti-T production and that there must be common determinants between them and T antigen of red blood cells. It is not sure whether the previously reported increase of anti-T levels in liver cirrhosis is in favour of the hypothesis that hypergammaglobulinaemia in this condition is due to failure of diseased liver to trap gut-derived antigens.