The effect of paclitaxel and of its solvent Cremophor EL, a polyoxyethylated castor oil on histamine release in rat peritoneal mast cells has been tested. Paclitaxel dissolved in Cremophor EL/ethanol and Cremophor EL alone induced a moderate histamine release; this secretory activity is provoked by the solvent Cremophor EL but is so scanty that it seems most unlikely to be responsible of the severe hypersensitivity reactions frequently caused by this drug. Since in a number of protocols paclitaxel is used in combination with anthracyclines, and since anthracyclines are well known histamine releasing agents, the effect of the combination of these substances was also studied. Cremophor EL did not increase the exocytotic activity of adriamycin; on the contrary, the release of histamine induced by this anthracycline was significantly limited. This is an interesting observation since it has been suggested that anthracycline-induced histamine release is involved in the pathogenesis of the cardiotoxicity caused by these drugs. Low concentrations of paclitaxel in Cremophor EL and Cremophor EL alone inhibited adriamycin uptake into the granules of mast cells, a process mediated by an active transport system, recently identified with the P-170 glycoprotein pump.