Following several reports of human allergic hypersensitivlty, the ability of 2,4‐D (2,4‐dichlorophenoxyacetic acid) to elicit 2,4‐D‐specific IgE antibodies and delayed‐type hypersensitivity in BALB/c mice was studied. 2,4‐D‐specific IgE antibodies were detected in mouse sera following the second intraperitoneal immunization with I, 10, or 100 μg 2,4‐D‐keyhold limpet hemocyanin conjugate with aluminum hydroxide adjuvant. Specific IgE was determined with the rat passive cutaneous anaphylaxis test using a conjugate of 2,4‐D with bovine serum albumin for challenge. The highest antibody titers and a measurable response in all mice were seen in the group that received 1 μg of 2,4‐D conjugate. Dinitrophenyl‐specific IgE was measured at all intervals examined in mice immunized with a dinitrophenyl‐keyhole limpet hemocyanin conjugate. 2,4‐D applied epicutaneously on 2 d or over 4 wk failed to elicit delayed‐type hypersensitivity as measured by change in ear thickness, incorporation of 5‐[125I] iodo‐2'‐deoxyuridine, or histology following challenge on the ear. No 2,4‐D‐specific IgE antibodies were detected in serum during the 4‐wk sensitization period. Similar treatment with a known sensitizer, dinitrofluorobenzene, produced delayed hypersensitivity. Following 4 wk of dosing, low titers of dinitrophenyl‐specific IgE antibodies were elicited.