Thymic nurse cells are isolated thymic epithelial/thymocyte complexes described in vitro. Their in vivo existence is controversial. We examined rat thymi in order to distinguish thymic nurse cells in vivo using the zinc iodide-osmium tetroxide (ZIO) fixation/staining technique. There were mainly two types of strongly ZIO reactive cells in the thymic cortex: macrophages and small clusters of thymic epithelial cells. The latter group was composed of thymic epithelial cells and intact thymocytes forming complexes. Thus we suggest that these structures are in situ equivalents of thymic nurse cells.