Abstract.Radioimmunoassays were prepared using two anti‐A and one anti‐B reagents. The specificity of the procedures was assessed with 13 artificial antigens. The amounts of A and B natural antigens in saliva of ABH secretors of known Lewis phenotype were measured with these assays. The results confirmed that the average amount of A antigen is lower in Lewis‐positive (Leb) than in Lewis‐negative (Led) donors and in A2than in A1donors. However, the differences among the four combined A and Lewis phenotypes were only supported by significantly lower amounts of A antigenic determinants in A2Lebas compared to the other three phenotypes (A1Leb, A1Ledand A2Led) that had similar amounts of A antigenic determinants. No Lewis‐related difference could be detected in the amounts of B antigens between BLeband BLeddonors. The results are discussed in terms of competition between A, B and Lewis‐gene‐specified enzymes for their common acceptors. The difference in the efficiency of the A2enzyme as compared to that of the A1enzyme is proposed as a possible explanation for the A1‐A2phenot
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