Electron micrsocopic investigation of the thymus of normal guinea pigs of both sexes, approximately 3 months of age, have shown that the organ contains at least two types of non-lymphoid cells: epithelial reticulum cells and so-called interdigitating cells. The latter have been observed in the inner part of the cortex and are morphologically characterized by an irregularly shaped or kidney-shaped nucleus, many tubulovesicular cytoplasmic structures accumulated mostly within the cytocentre as well as by finger-like protrusions of the cytoplasm. These interdigitating cells are in close contant with neighbouring lymphocytes with partly destroyed pycnotic nucleus.