The capacity of cells from thymus, liver, spleen, mesenteric lymph nodes, peripheral blood and bone marrow to respond to in vitro phytolectin [pokeweed mitogen, phytohemagglutinin-P, concanavalin A] and allogeneic lymphocyte-stimulation was determined in 16 pony fetuses 61-200 days old (gestational age). Phytolectin-responsive cells were detected in the thymus at the 80th gestational day, peripheral blood at 120 days, lymph node at 160 days and spleen at 200 days. Mixed lymphocyte culture-responsive cells were detected in thymus at 100 days and in the spleen at 200 days (gestational age). IgM and IgG were quantitated by radioimmunoassay. They were detected in fetuses prior to 200 days of age. All of 50 normal newborn foals had detectable quantities of IgM (165 .+-. 56 .mu.g/ml of serum). Quantitites of IgG in normal newborn foal serum were lower and more variable. The minimal-maximal concentrations of IgG were 2-170 .mu.g/ml of serum with a mean and SD of 51 .+-. 49 .mu.g/ml. Apparently, functional T lymphocytes are present in the fetus by 100 days (gestational age), functional B lymphocytes are present by 200 days and foals are immunocompetent before birth.