Histidine decarboxylase activity and histamine levels of peritoneal mast cells were enhanced 4 days after intraperitoneal Haemophilus influenzae vaccination of rats. Incubation of the cells with propranolol (3.4 x 10(-4) M) resulted in histamine release and an increased histidine decarboxylase activity. Histidine decarboxylase activity and histamine release were more increased in the presence of propranolol in mast cells obtained from H. influenzae-vaccinated rats. An increased mediator release is also suggested by the increase of the number of peritoneal eosinophils. These data might explain the earlier observed enhanced plasma and lung histamine levels in H. influenzae-vaccinated rats.