A sensitive method for quantifying histamine is the assay based on the fluorescence of the product obtained after reacting the amine with ortho-phthalaldehyde. The presence of dimethyl sulfoxide (DMSO) even in concentrations as low as 1% decreases the yield of fluorescence. Neglecting this quenching leads to the erroneous conclusion that DMSO in a concentration dependent manner inhibits the thapsigargin induced histamine release from peritoneal rat mast cells. Apart from the release induced by the selective calcium ion mobilizers thapsigargin and ionophore A23147, DMSO potently inhibits the response to other secretagogues.