The distribution of lysozyme (muramidase) within eosinophil leukocytes situated in the lamina propria of human colon was studied by immunoelectron microscopy using a range of standard techniques. Tissue processed in a variety of glutaraldehyde- or paraformaldehyde-based fixatives was partially dehydrated and embedded in the acrylic resin LR White. Tissue thus treated showed lysozyme in pale cytoplasmic granules and the matrix of specific granules, but not in their crystalloids. Trypsinization of sections had little effect on this result, and tissues fixed in glutaraldehyde and embedded in Araldite showed a low level of reactivity with a similar distribution. After etching LR or Araldite sections with sodium metaperiodate, the pale granules and specific granule matrices became negative for lysozyme and the crystalloids became positive. Because crystalloids also were labeled with noral rabbit immunoglobulin after etching, this apparent redistribution of label could be due to nonspecific binding rather than exposure of masked epitope.