A small population of normal thymic lymphocytes, like the majority of thymic leukemia cells, formed cellular complexes with thymic epithelium-like stromal cells in pseudoemperipolesis. The properties of the complex-forming cells in preleukemic AKR thymus were analyzed after separation of the cells into subpopulations on the basis of cell size and surface antigen expression by using a fluorescence-activated cell sorter. Complex-forming ability was associated with large cells and the following phenotypes: high Thy-1.1, low brain associated theta antigen, high H-2Kk, high Lyt-1, high gp70 and Ia negativity. These properties coincide in general with those of blast cells in the subcapsular zone of the thymus, which have been shown to consist mostly of complex-forming cells. The possible significance of complex formation of normal and leukemic thymocytes with thymic stromal cells is discussed.