Pregnant 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 by Escherichia coli lipopolysaccharide, 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 alterations in the immune system.