Total and allergen-specific serum IgE were measured in relation to allergic response pattern before and after bone marrow transplant (BMT) in seven sibling donor/recipient pairs. Two non-atopic recipients developed persistently raised total serum IgE levels but no apparent allergic response after BMT from non-atopic donors; three non-atopic recipients showed raised total serum IgE after BMT, with allergen-specific IgE to the same allergens as their respective atopic donors. A penicillin-tolerant recipient showed clinical sensitivity and specific IgE to penicillin after BMT from a penicillin-sensitive donor, but with this case both donor and recipient showed raised serum IgE levels. One atopic recipient showed decreased total IgE after BMT from a mildly atopic donor. These allergic response patterns could occur as a result of repopulation in the recipient with IgE-specific T lymphocytes having similar regulatory influences as in the donor. The pattern of acquired responses would also be consistent with reconstitution by primed B lymphocytes of donor origin.