The degree and type of bacterial colonization was similar in achlorhydric patients with either severe primary hypogammaglobulinaemia or classical pernicious anaemia. This suggests that antibodies do not play a significant part in controlling the gastric flora in achlorhydric patients. The nitrite concentration in gastric juice was higher in the hypogammaglobulinaemia patients, raising the possibility that these patients may have very high levels of nitrite-derived mutagenic compounds in their gastric juice. This may account for the high incidence of gastric cancer in these patients.