Six monoclonal antibodies reacting with equine T lymphocytes at different stages of maturation were selected from antibodies produced against lymphoid cell preparations. EqT12 and EqT13 antibodies identified subsets of cortical thymocytes with high terminal deoxynucleotidyltransferase (TdT) activity and no phytolectin responsiveness. EqT12+ thymocytes were scattered throughout the cortex while EqT13+ thymocytes were located in the subcapsular cortex. EqT12 bound to small numbers of bone marrow cells, splenocytes, and circulating lymphoid cells, but not to mature T lymphocytes. EqT13 bound to very small numbers of bone marrow cells but not to more mature lymphocytes. EqT6 and EqT7 reacted with a large population of cortical thymocytes with high TdT activity and no phytolectin responsiveness. EqT2 and EqT3 bound primarily to medullary thymocytes with low TdT activity. Eq2+ thymocytes responded to phytolectin stimulation while EqT3+ thymocytes did not. EqT2 and EqT3 bound to 33% and 91% of circulating T lymphocytes, respectively. The T lymphocytes bound by both antibodies included cells capable of suppressing a mixed lymphocyte reaction. Thus, EqT12 and EqT13 identify cells with the functional characteristics of prothymocytes. EqT6 and EqT7 identify resident cortical thymocytes, and EqT2 and EqT3 identify a subpopulation of mature T lymphocytes and all mature T lymphocytes, respectively.