In the sera of patients who developed asthmatic symptoms after exposure to chloramine-T, specific IgE antibodies are demonstrated by a polystyrene tube radioimmunoassay. These antibodies are directed against human serum albumin treated with chloramine-T. Specific antibodies of subclasses IgG1, IgG3 and IgG4 were not found. Histamine release in response to chloramine-T-treated human serum albumin was demonstrated using the peripheral blood leukocytes of one of the patients. To define the antigenic determinant to which the specific IgE antibodies are directed, radioimmunoassay and radioimmunoassay-inhibition studies were performed using several autologous and heterologous chloramine-T-treated proteins. Moreover, the inhibition capacities of chloramine-T and structurally related compounds were also determined. The results of these experiments indicate that the antigenic determinant is formed, at least in part, by the p-toluenesulfonyl group of the chloramine-T molecule.