Aboriginal secretors synthesize greater concentrations of H, A and Lea antigens than do Caucasians but, in Ax secretors, most H determinants are converted to A. Since Aboriginal secretors have concentrations of Lea in the saliva similar to those of Caucasian non-secretors, it is suggested that the red cell phenotype, Le(a+b+), observed in over 10% of Aborigines, is due to the high concentration of Lea which is not converted to Leb. Aboriginal A1secretors have significantly greater concentrations of Lea in their salivas than have·secretors. This observation has led to the proposal that, in the genetically-controlled biosynthetic pathways leading to the production of Lewis and ABH antigens in saliva, the genes add their specificities in the order H, A (and/or presumably B), Le.