We have studied the distribution of mast cells (MC) in different tissues of the guinea pig (GP), and certain aspects of their histochemical and cytochemical properties. In preparations fixed with Carnoy's fluid, MC were extremely well-preserved and were detected in all tissue sections and cytocentrifuge smears obtained from enzymically dispersed cells after staining with alcian blue at pH 0.5-2.2. In formol-saline-fixed preparations, there was a substantial reduction in MC counts. Most of the tissue MC were located interstitially and in the perivascular connective tissue. A small proportion of MC was located in certain organ- or tissue-specific structures. When counted manually, MC comprised 0.43-3.1% of the cell population obtained by enzymic dispersion. This proportion was higher than that obtained by the automated Technicon H6000 basophil system, but the two counting systems confirmed organ differences in MC counts. MC in different GP tissues varied in size, morphology, histamine content and degree of susceptibility to formol-saline fixation. The granules of GP connective tissue MC differed from those of rat peritoneum in failing to take the fluorescent stain berberine sulphate which has greater affinity for the highly sulphated glycosaminoglycans such as heparin, and in having some morphological and histochemical similarity to 'mucosal' MC.