The fine structural features of the intraepithelial lymphocytes in the human small intestine include an often irregular outline, sparse cytoplasmic organelles and a compact nucleus. Desmosomal attachments are not seen between lymphocytes and adjacent epithelial cells. Intraepithelial lymphocytes in general are larger than the average peripheral lymphocyte. No significant fine structural differences are seen between intraepithelial lymphocytes in normal intestine and those in celiac disease. Intraepithelial eosinophils and mast cells are also described, their identity being established by their characteristic inclusions. Attention is drawn to projections of the epithelial cells through breaks in the basal lamina. These herniations are thought to be caused by the passage of migrating lymphocytes.