Human mast cells were obtained from adenoids and mesentery by enzymatic dispersion of the tissues with the enzyme collagenase. The digestion of the tissues resulted in a cell suspension which contained 1 - 2% mast cells. 37,3% (adenoids) and 33,4% (mesentery) of total histamine initially present in the tissues was recovered in the dispersed cell suspensions. More than 90% of the cells were viable. The adenoidal mast cells could be sensitized passively in vitro with homologous reaginic serum and released histamine after challenge with specific antigen. Both populations of mast cells were sensitive to the action of anti-human IgE: the reversed anaphylaxis with anti-IgE was higher in mesenteric mast cells. Both examined mast cell populations were sensitive to the challenge with polymyxin B, concanavalin A and ionophore A23187, however, histamine release was only up to 10% and 20% for adenoidal and mesenteric cells, respectively. Only mesenteric mast cells responded to the action of compound 48/80. Histamine release induced by polymyxin B, was rapid (maximal release within 5 min), maximal in the presence of 3 mM extracellular calcium ions (but also occured in the absence of the cation).