The phenomenon of autorosette formation was investigated by incubating the peripheral blood mononuclear cells of 5 normal persons with a series of monoclonal antibodies and an FITC-labelled second layer before allowing them to bind to autologous red blood cells. The antibodies used were OKT3, OKT4, OKT5, OKM1 and OKIa1, defining mature T cells, the inducer and suppressor subsets of T cells, monocytes and Ia antigen bearing cells, respectively. By this procedure the percentage of autorosette-forming cells was only slightly decreased. With this double marker analysis, the percentage of fluorescent cells among cells forming autologous rosettes was approximately the same as in the total mononuclear cell population before rosetting. In conclusion we could demonstrate that autologous rosettes in man may be formed by T and probably B lymphocytes as well as by monocytes and that there is no preference for the inducer and suppressor subsets of T lymphocytes.