The procedure tor carrying out the Leucocyte Ascorbic Acid Uptake Direct Antigen Challenge Test (LAADACT) is described. Leucocytes from normal individuals, when incubated in a buffered medium containing ascorbic acid, increase their ascorbic acid concentration by about 80%. When leucocytes from atopic individuals are incubated in a medium containing the antigen to which they are sensitive, as shown by positive skin tests, the leucocyte uptake of ascorbic acid is significantly reduced. Addition of antigen, to which atopic or normal individuals are not sensitive, to the incubation mixture does not reduce leucocyte ascorbic acid uptake. Measurement of ascorbic-acid uptake into leucocytes is a relatively simple, routine, laboratory procedure. The LAADACT, therefore, provides a quick and accurate blood test for diagnosing sensitivity to specific antigens, and measuring relative anligenic sensitivities. The underlying mechanism of the LAADACT is discussed.