Experiments in vitro have shown that bacteria induce histamine release and potentiate IgE-mediated histamine release. In the present study, these events were examined in allergic patients by an in vivo model using nasal challenge. Nasal spray with 56 mg Staphylococcus aureus triggered histamine release in the nasal cavity in 4 of 13 patients, whereas no response was obtained by 28 and 112 mg bacterium. These findings indicate that bacteria containing peptidoglycan may release histamine in vivo. To avoid allergens, we used anti-IgE antiserum. In six patients nasal challenge with anti-IgE (112000 IU) caused an, increased histamine level in the nasal fluid which was not obtained by a control preparation without anti-IgE antibodies. The IgE-mediated mediator release was potentiated by the bacterium in only 2 of 22 patients.