Six week old female BDF1 mice were administered the n-butylester of 2,4-dichlorophenoxyacetic acid (2,4-D) by gastric intubation at dosages ranging from 0 to 200 mg/kg (2,4-D content) in the acute studies and 0 to 100 mg/kg in the subacute studies. Following acute exposure, the antibody production against sheep red blood cells and the induction of DNA synthesis with Escherichia coli lipopolysaccharide, a B-lymphocyte mitogen, were enhanced. Comparable T-lymphocyte mitogen responses induced by concanavalin A, a T-lymphocyte mitogen, were not affected by 2,4-D, though 2,4-D did exhibit a weak, non-specific, dose-related mitogenic effect. Subacute exposure to the 2,4-D ester did not alter antibody production. B-lymphocyte mitogen responses were enhanced, though a linear dose-response relationship was not observed. Histopathological alterations in the brain and spinal cord and clinical symptoms of toxicity were observed in the 200 mg/kg treatment group (acute exposure). Similar effects were not seen in the subacute study. The immunostimulatory effects of 2,4-D were observed at relatively high exposures. It is unlikely that these immune alterations will have any major toxicological significance in agricultural communities utilizing 2,4-D herbicides, though further studies with commercial grade formulations which may contain other compounds in addition to the pure 2,4-D esters must be evaluated at similar levels of exposure.