Naturally occurring plant flavonoids, normal dietary constituents, affect a variety of cell activation phenomena including the secretion of histamine from human basophils stimulated by a variety of agents (antigen, anti-IgE, concanavalin A, ionophore A23187, formyl-Met-Leu-Phe and tetradecanoyl phorbol acetate). Variable profiles of inhibition are seen depending on the nature of the stimulus and the chemical structure of flavonoids.