Haemophilus influenza and its extracellular produets (EP) ditl not release histamine from basophil leukocytes in cell suspensions from normal individuals, patients with chronic bronchitis or patients allergic to either house dust mite, grass pollen, eat dander or to their own bacteria. However, the EP was found to enhance their basophil histamine release. IgE‐mediated histamine release was examined by stimulation of the cells with anti‐IgE or the specific allergens, and non‐immunological histamine release by stimulating the cells with the calcium ionophore A23187 or Staphylococcus aureus. In all the experiments EP caused a significant increase in the histamine release. When H. influenzae endotoxins were removed from the EP, the potentiating effect of EP was completely abolished, whereas heating (80°C. 30 min) or treatment of EP with proteinase did not influence the potentiating effect. These results indicate that H. influenzae endotoxin potentiates histamine release caused by IgE‐mediated reactions or by non‐immunological mechanisms.