As recently reported, Sarcocystis gigantea lectin (SGL) is a powerful mitogen and a polyclonal activator (Syn. S. ovifelis) of human peripheral B-cells. In the present study we investigated the reactivity of human T-helper (CD4) and T-suppressor (CD8) cells to SGL. Mononuclear cells (MNCs) from five newborns and six adults were examined cytofluorometrically for the expression of cell-surface differentiation and activation antigens using a set of seven monoclonal antibodies. In all, 96% of cord-blood and 81% of adult CD4 cells expressed receptors for interleukin-2 (Tac+) after 64 and 164 h microgram/ml). The percentages of neonatal and adult Tac+ CD8 cells amounted to 67% and 59%, respectively. Major histocompatibility complex (MHC) class II antigens identified using the monoclonal anti-human leukocyte antigen (HLA)-DR antibody L243 were expressed on 30% (CD4) and 44% (CD8) of adult T-cells. Neonatal HLA-DR+ T-lymphocytes were not detectable. In parallel, functional tests were performed to examine cell proliferation and MNC antibody production.