SUMMARYWe studied structural and functional characteristics of lymphocytes from adult and fetal baboons (Papio cynocephalus). Flow cytometry with monoclonal antibodies to human lymphocyte antigens and plant lectins was used to define expression of surface antigens on lymphocytes from adult and 140 day fetal baboons (term = 180 days). Major T cell antigenic determinants on adult and fetal baboon lymphocytes were the Tp50, Tp32‐45, and p45 glycoproteins detected by monoclonal reagents T11, OKT8, and OKT10 respectively. Baboon T lymphocytes did not react with the OKT3/anti‐Leu4 or OKT4/ anti‐Leu3a reagents which detect, respectively, Tp19‐29 and Tp55, major surface glycoproteins on human T lymphocytes. OKT6, which identifies the human TL antigen equivalent on thymocytes, did not react with baboon thymocytes. These data demonstrate major evolutionary divergence between human and baboon T lymphocytes. By contrast, baboon lymphocytes resembled human peripheral lymphocytes in reactivities with several non‐T cell reagents. Lectin binding studies revealed substantially fewer peanut agglutinin‐and wheat germ agglutinin‐binding cells in suspensions of baboon fetal splenocytes and adult peripheral lymphocytes compared with fetal thymocytes. Thereffore, maturation of baboon T lymphocytes is associated with loss of surface carbohydrate structures that bind these lectins. Adult and fetal baboon lymphocytes resembled human and murine lymphocytes in their capabilities to respond to mitogens and to produce interleukin‐2. As in oter species, adult, but not fetal baboon lymphocytes, mediated NK activity against a variety of nucleated target cells. Despite divergence in lymphocyte antigen epression, babbon lymphocyte functional development colsely parallels that