2,4-D-specific IgE antibodies were detected in serum of BALB/c mice using the rat passive cutaneous anaphylaxis (PCA) test following the second intraperitoneal immunization with 1, 10, or 100 ug of 2,4-D0keyhole limpet hemocyanin (KLH) conjugate administered with aluminum hydroxide adjuvant. The groups that received 1 ug of 2,4-D conjugate had the highest antibody titers (geometric mean of 60). A paper radioallergosorbent test (PRAST) was developed for determination of 2,4-D-specific IgE. The PRAST was equally as sensitive as and showed a positive correlation with the PCA assay. The anhydride of the diacid metabolite of malathion (MMA) coupled to KLH elicited MMA-specific IgE following secondary immunization with 10 and 100 ug and tertiary immunizations with 1, 10, and 100 ug of conjugate. The highest PCA titers (geometric means of 208 and 195) were found after three immunizations with 10 or 100 ug of conjugate, respectively. A PRAST for MMA-specific IgE was developed and yielded results equivelant to those obtained using the PCA procedure. Concurrently with these studies, dinitrophenyl-specific IgE elicited with 1 ug of dinitrophenyl-KLH conjugate was measured by the PCA test at all intervals examined. 2,4-D and malathion applied epicutaneously on two days or over four weeks failed to elicit delayed-type hypersensitivity (DTH). Cptafol produced DTH responses at both dose levels tested. Following two applications, ear thickness, incorporation of 5-[125I]iodo-2'deoxyuridine-labelled cells, and histology of ears indicated swelling and cellular infiltration. Multiple sensitizations over the period of a month also produced DTH as indicated by increases in ear thicknesses. Mice pretreated with cyclophosphamide produced larger responses 24 hours post-challenge but equivalent responses at 48 hours as compared to mice pretreated with saline. Sensitization with a known sensitizer, dinitrofluorobenzene, also elicited DTH. Neither 2,4-D- nor MMA-specific IgE antibodies were detected in serum during the four week epicutaneous sensitization period. Low titers of dinitrophenyl-specific IgE were elicited in mic treated with dinitrofluorobenzene.