The mast cell response in the mucosa and connective tissue of 36 jejunal biopsies of patients with clinically diagnosed trichinellosis, teniasis and lambliasis has been studied. Biopsy material was fixed in standard formalin or Carnoy's fixative, enabling differentiation between mucosal mast cells (MMC) and connective tissue mast cells (CTMC). With both fixatives CTMC could equally well be recognized. With Carnoy's fixative an additional population of mast cells (MMC) could be visualized both in the mucosa and the connective tissue. In the mucosa small mucosal mast cells were observed as well. Compared to the numbers of mast cells in the mucosa and the connective tissue of teniasis and lambliasis patients, the number of mast cells in trichinellosis patients only visualized using Carnoy's fixative was markedly higher. It was concluded that also in man trichinellosis is accompanied by an increase of cells with MMC characteristics. Further studies are needed to clarify the morphological and histochemical features of these cells and their possible role in this parasitic infection.