AbstractPregnant CD‐1 mice were administered the n‐butylester of 2,4‐dichlorophenoxyacetic acid (2,4‐D) by gastric intubation on day 11 of gestation at dosages ranging from 0 to 200 mg/kg (2,4‐D content). The immune response in the female offspring was elevated at 6 weeks of age. The humoral immune response, antibody production against sheep red blood cells, was not altered by 2,4‐D ester exposure during gestation. The mitogen responses of lymphocytes induced by concanavalin A, a T‐lymphocyte mitogen, or byEscherichia colilipopolysaccharide, a B‐lymphocyte mitogen, were reduced in the highest exposure group (200 mg/kg), although the T‐lymphocyte suppression was not statistically significant. A similar response pattern was observed in the background nonstimulated lymphocyte cultures, suggesting that the suppression was a generalized lymphocyte abnormality. Evaluation of the mitogen responses using stimulation indices to correct for the variable background responses demonstrated that 2,4‐D produced no net suppressive effect in any of the treatment groups. Since in utero 2,4‐D ester exposure produced no alterations in humoral immunity and only subtle effects on lymphocyte blastogenesis, it is unlikely to be of any immunotoxicological or immunoteratological significance. Further studies investigating commercial‐grade 2,4‐D formulations are necessary since these formulations contain other components that may potentially induce alterat