The capacity of cells from thymus, liver, spleen, mesenteric lymph nodes, peripheral blood, and bone marrow to respond to in vitro phytolectin and allogeneic lymphocyte-stimulation was determined in 16 pony fetuses 61 to 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). Immunoglobulins (Ig) M 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 micrograms/ml of serum). Quantities of IgG in normal newborn foal serum were lower and more variable. The minimal-maximal concentrations of IgG were 2 to 170 micrograms/ml of serum with a mean and SD of 51 +/- 49 micrograms/ml. The results indicate that (1) functional T lymphocytes are present in the fetus by 100 days (gestational age), (2) functional B lymphocytes are present by 200 days, and (3) that foals are immunocompetent before birth.