Introduction: Various drugs and other low molecular weight chemicals can induce T cell-dependent B cell activation which may cause allergic or autoimmune-like diseases. Because the nature of the relevant (neo-) antigens or hapten-carrier complexes are generally not (in case autoimmunity) or not always (in case of allergy) known, we explored the potential use of well-defined reporter antigens to determine T cell-dependent B cell activation by chemicals. TNP-ficoll and TNP-OVA were used as reporter antigens because they are recognized by the same TNP-specific B cells. For specific antibody production, however, distinct co-stimulation is required.Materials and Methods: Various compounds were coinjected with the reporter antigens into the footpad of female BALB/c (wild-type, nu/+ or nu/nu) mice. Seven days later the number of TNP-specific AFC (IgG1) in suspensions of popliteal lymph nodes was measured with ELISPOT assay. In similarly treated mice, kinetics of of TNP-specific antibody titers were followed for 4-5 weeks. In the same mice, memory responses (DTH and specific antibodies) were measured 6 days after challenge with the reporter antigens alone.Results: It was found that the autoimmunogens HgCl 2 , phenytoin, nitrofurantoin and D-penicillamine and the allergens DNCB, TMA and tulipaline stimulated IgG1 production to both antigens, the macrophage activators incomplete Freund's adjuvant and silica and the irritants glutaric anhydride, CuCl 2 , and DMSO to TNP-OVA only, and hydroxyl-amino procainamide to TNP-ficoll alone. The diabetogen streptozotocin did not enhance IgG1 production, but enhanced a cellular response (IgG2a, CD8 + cells, IFNγ) instead. Ethanol and acetone did not influence the responses. IgG1 production to TNP-ficoll was local, transient and did not always require T cell-help. In contrast, responses to TNP-OVA could be measured in serum, led to specific memory and were strictly T cell-dependent.Conclusion: Results demonstrate that specific antibody production to reporter antigens very sensitively indicates distinct immunostimulatory effects of chemicals and can distinguish formation of neoantigens and haptenization from mere irritating effects. Moreover, with TNP-OVA the kinetics and regulation of chemically enhanced immune responses can be studied in mice strains with varying sensitivity for the adverse effects, without the need to know the relevant neo-antigens.