['Morphine co-injection has anti-inflammatory effects on zymosan-induced peritonitis in several strains of mice except that of CBA. As peritoneal mast cells (pMCs) are much more numerous in CBA mice than in SWISS mice, therole of pMCs in morphine-modulated zymosan peritonitis is compared in CBA and SWISS males.', 'pMCs were treated in vitro with morphine or C48/80 for comparison of histamine release. In vivo accumulation of leukocytes and histamine in peritoneal exudate were recorded after intraperitoneal injection with morphine, zymosan, or zymosan plus morphine.'] Are mast cells responsible for the lack of anti-inflammatory effects of morphine in CBA mice?