An immunohistochemical double-labelling technique for the simultaneous identification of mast cells containing tryptase alone (MCT) or chymase together with tryptase (MCTC) was evaluated quantitatively using two monoclonal antibodies, mAb 1222A (antitryptase) and mAb 1254B (antichymase). Saturation conditions were established for the binding of the antibodies to the mast cell enzymes by counting labelled mast cells in consecutive sections of normal human intestine incubated with serial dilutions of the antibodies. When, under such conditions, the antitryptase was applied after saturation with mAb 1254B, the reproducibility of the double-labelling procedure was excellent. MCT were located preferentially in the intestinal mucosa but, in contrast to what has previously been reported, they were not the predominant type of mast cell at this site. The percentage of MCT of the total number of immunopositive mast cells varied considerably in the colonic mucosa (7-67%, average 30%), while this was not the case in the small intestinal mucosa (5-26%, average 10%). Mast cell chymase, unlike tryptase, was not recognized by the antichymase antibody after aldehyde fixation and a higher apparent fraction of MCT therefore occurred after double labelling. These findings suggest that the proteinase composition of human mast cells, unlike that of murine mast cells, should not be taken as evidence of phenotypic heterogeneity. Taken together with previous observations, they suggest instead that the lack of chymase may be related to functional activity or stage of maturation of the mast cells.