The IgE antibody responses of inbred BALB/c mice to subcutaneously injected antigens, given without adjuvant, were studied. Cocksfoot grass pollen extract (GPE) (100-100,000 pnu) induced no detectable primary IgE antibody responses but following second and subsequent injections, substantial secondary responses were produced. Weekly injections for 9 weeks of doses of doses of 0.1-10,000 pnu GPE all produced similar IgE antibody titres as measured by 24 h passive cutaneous anaphylaxis (PCA) in rats challenged with 16,000 pnu GPE. A similar schedule of injections of doses of 10(-10)-10(-5) g of DNP-OA also produced substantial and similar anti-DNP IgE antibody titres as measured by PCA in rats challenged with 1 mg DNP-BSA. As the challenging dose of DNP-BSA used in the induction of PCA was reduced progressively however, the PCA titres of the mouse sera showed more and more dependence on immunizing antigen dose. At limiting doses of antigen in PCA, the serum titres rose to a peak and then declined, as the immunizing dose of DNP-OA used in the mice was progressively increased. The possible implications of these results with regard to our understanding of the IgE antibody response in atopic patients is discussed.