Several different grades of T-system impairment were studied for their effects on the total serum IgE concentration in BALB/c mice. Homozygous athymic nu/nu mice and their heterozygous nu/+ littermates were compared for serum IgE levels while kept under either barrier-maintained or conventional conditions. The results show a paradox between the T-cell dependency of the IgE immune response and the increased levels of serum IgE in the absence of T cells. Both barrier-maintained and conventionalized nu/nu mice have at least twofold increased serum IgE levels as compared to nu/+ mice. With age, IgE levels increased faster and reached higher plateau values in nu/nu than nu/+ mice. Moreover, after adult thymectomy of BALB/c mice the serum IgE levels increased up to 15-fold at 4 months of age, while infusion of immunocompetent T cells in nude mice resulted in a 2- to 5-fold decrease of the IgE level.