The role of antigens of Brugia malayi adult worms in induction of histamine release from mast cells was studied. Both peritoneal and lung mast cells were passively sensitized using immune serum collected from Mastomys natelensis on different days after infection with B. malayi. A significant release of histamine both with crude worm and 60-kD antigens was shown. However, the role of the 43-kD antigen in histamine release was comparable to that of control. When the sera were heat inactivated, the histamine release was minimal, thus indicating the heat-labile nature of the antibodies. Furthermore, the responses of peritoneal and lung mast cells to filarial antigens were similar.