A method is described for determining whether particular monoclonal antibodies are specific for carbohydrate or non-carbohydrate antigenic determinants. In a model system consisting of the Lewis a human blood group determinant attached to either protein or lipid, mild periodate oxidation destroyed the carbohydrate determinant without altering protein or lipid epitopes. The technique was readily applied to antigens bound to plastic wells for ELISA, to nitrocellulose sheets for Western blots, and to thin layer chromatography (TLC) plates for TLC immunostaining. Mild periodate oxidation can prove useful during the early stages of hybridoma screening in order to select for or against anti-carbohydrate antibodies.