The histochemical characteristics of a sulphonated polystyrene resin were investigated and compared with those of sulphonic acid residues derived from intrinsic tissue components. The major features of the resin particles were their direct Schiff positivity and basophilia. Unlike the tissue sulphonic acids, the resin particles failed to stain with alcian blue. Because of their acid nature, the particles may be responsible for the onset of an aspiration pneumonia if they are accidentally inhaled by debilitated patients.