A method is described for measuring rapid, specific, and saturable binding of the skin irritant and tumour-promoting secretagogue thapsigargin (sesquiterpene lactone) to the microsomal fraction from mouse brain. Employing the tritium-labelled compound its apparent dissociation constant, K_d, and the maximal amount of binding B_max are shown to be 9.8 n M and 1.9 pmol/mg protein respectively. Such a K_d for thapsigargin is similar to (a) its IC₅₀ value for inhibiting Ca²⁺ uptake in the microsomal fraction from rat brain and (b) its EC₅₀ values for inducing a rise in the cytoplasmic Ca²⁺ concentration of human platelets and histamine release from rat peritoneal mast cells. A positive correlation is found between the binding affinities of thapsigargin, thapsitranstagin, and trilobolide, their potencies as secretagogues and their lipophilicities. This correlation does not extend to the skin-irritant activities of the compounds thus emphasizing that their mechanism of action is unlike that of 12-O-tetradecanoylphorbol 13-acetate.