The fluorescence-activated cell sorter (FACS) was used to examine the expression of several alloantigens on T cells: Thy-1, Lyt-1.1, Lyt-2.1, Ly-5, Ly-6 and Ly-7. Antibodies secreted by monoclonal cell lines were used to characterize the Thy-1:2, Lyt-1.1 and Lyt-2.1 antigenic determinants, whereas Ly-5.1, Ly-6.2 and Ly-7.2 determinants were identified with alloantisera raised by conventional hyperimmunization. Using indirect immunofluorescence with fluorescein isothiocyanate-conjugated reagents, the profiles obtained with the FACS demonstrated that: (a) the expression of Thy-1 is greater on normal thymocytes than on cortisone-resistant thymocytes (CRT), lymph node and spleen cells; (b) in contrast to Thy-1, the expression of Ly-1 on normal thymocytes is less than on CRT, lymph node and spleen cells; (c) about 90% of normal thymocytes abut less than 50% of CRT, spleen and lympho node T cells are Lyt-2+ and (d) although weakly expressed on normal thymocytes, the amount of Ly-6 and Ly-7 present on peripheral T cells was considerably increased. The significance of these findings and the potential for further analysis of T cell developmental pathways is discussed.