Basophil histamine release was studied in leukocyte suspensions from normal individuals and from patients allergic to house dust mite or birch pollen. Mediator release caused by IgE-mediated reactions was examined by stimulating the cells with anti-IgE or specific antigens, and the calcium ionophore A23187 was used for a non-immunological histamine release. In all experiments influenza A virus caused a synergic enhancement of the mediator release and the potentiation was abolished by galactose (10(-7) to 10(-6) M) and by 10(-6) to 10(-5) M of N-acetylglucosamine, alpha-methyl-D-glucoside, alpha-methyl-D-mannoside, N-acetylneuraminic acid and lactose, but not by glucose. Wash-out experiments show that the sugars prevent the aggravation of mediator release by a binding of sugar to the basophil cell membrane, thereby causing a blockade of binding sites responsible for the potentiating effect of virus.