A new inhibitor of IgE-mediated allergic reactions has been studied in rats. This new inhibitor is chemically quite different from cromoglycate and shows qualitative and quantitative advantages over it. This new inhibitor possesses no bronchodilator or end-organ antagonism activity, but as cromoglycate acts by inhibition of mediator release. Multiple doses in rats lead to tachyphylaxis. This effect is reversed both by decreasing the first dose and increasing the time between doses.