A number of enzymes were screened for their ability to dissociate mesenteric tissues from the rat and guinea pig into their component cells. The bacterial enzyme collagenase was found to be the most satisfactory agent and a procedure based on the use of this protease was developed. The resulting suspensions contained 1–2% free mast cells and exhibited a low (ca. 5%) spontaneous release of histamine. The tissue cells contained less histamine than rat peritoneal mast cells and the guinea pig cells were smaller in size. Cells obtained from actively sensitized animals responded to antigenic challenge more strongly than the chopped tissue indicating that they were functionally intact. Rat mesenteric cells could be passively sensitized with homologous reaginic antibody and also responded to anti-rat IgE. The immunologically induced releases from rat mesenteric and peritoneal cells showed differing sensitivities to potentiation by phosphatidyl serine but the responses were directly comparable in the absence of this effect. Rat mesenteric cells also responded, but less effectively than the peritoneal cells, to the ionophore A23187, concanavalin A, ATP and basic secretagogues. They were, however, essentially refractory to the action of dextran. In contrast, guinea pig mast cells responded strongly only to the ionophore and weakly or not at all to the other agents. These results indicate marked inter-and intra-species differences in the reactivity of mast cells and suggest that rat peritoneal cells should not be used as the sole model for studying histamine secretion.