A new 28 amino acid peptide, we recently isolated from the venom of the bumblebee Megabombus pennsylvanicus. has been characterized. The peptide, Met-Cys-Ile-Cys-Lys-Asn-Gly-Lys-Pro-Leu-Pro-Gly-Phe-Ile-Gly-Lys-Ile-Cys- Arg-Lys-Ile-Cys-Met-Met-Gln-Gln-Thr-His(NH2), has been named bumblebee mast cell degranulating (MCD) peptide due to its ability to degranulate rat peritoneal mast cells, and its resemblance to the bee venom MCD peptide. Bumblebee MCD peptide, unlike bombolitins, the other mast cell degranulating heptadecapeptides of bumblebee venom, is not lytic and releases histamine at a dose as low as 0.05 micrograms/ml (1.6 X 10(-8) M).