Sensitizing chemicals can cause different forms of allergy, allergic contact dermatitis, or sensitization of the respiratory tract. These discrete types of chemicals induce in mice qualitatively divergent immune responses; contact allergens provoke preferential type 1 responses, whereas respiratory allergens stimulate selective type 2 responses. We have questioned whether the ability of chemicals to initiate polarized immune responses is in part a function of the nature of their association with protein. Cytokine secretion profiles provoked following topical exposure of BALB/c mice to dinitrochlorobenzene (DNCB), dinitrofluorobenzene (DNFB), fluorescein isothiocyanate (FITC), trimellitic anhydride (TMA), and dinitrobenzene-sulfonyl chloride (DNBSCl) were compared with the distribution of covalent binding to U937 cells and/or to serum proteins in vitro. DNCB and DNFB each provoked a type 1 cytokine secretion profile, with high levels of IFN-γ, but relatively low levels of type 2 cytokines IL-4, -5, and -10. The converse selective type 2 phenotype was seen following equivalent exposure to TMA, FITC, or DNBSCl. Each chemical bound covalently to U937 cells and to serum proteins, when incubated with cells or serum alone. When incubated with cells and serum together, DNCB and DNFB bound selectively to cellular protein, whereas TMA, FITC, and DNBSCl bound selectively to serum. These investigations show that the distribution of antigen formation of chemical allergens in an in vitro model system segregates with the type of cytokines secreted from activated lymph node cells in an in vivo mouse model. Chemical allergens that stimulate type 1 cytokine secretion profiles bind selectively to cellular proteins, whereas others that provoke type 2 cytokine profiles bind preferentially to serum proteins.