Summary1. The sib pair data suggest that the amount of A, B or H antigen secreted by an individual is in part inherited.2. Most of the inherited component of the variance in antigen secreted appears to be polygenic.3. The data confirm that group O people secrete more H substance than do group A people who, in turn, secrete more than group B people, and that group AB people secrete least of all. This suggests that there may be a difference between homozygotes and heterozygotes of groups A and B with regard to the amount of H secreted.4. The ratio of A:H secreted appears to be in part controlled genetically.5. The distribution of the ratio of A: H substance in group A secretors indicates that many of the so‐called aberrant secretors are really the arbitrarily chosen extremes of a continuous and unimodal distribution of ratios. If this be so, aberrant secretors should be defined as individuals whose A: H or B: H ratio falls more than a given number of standard deviations from the mean of the control group. Such a definition would allow people in different places, using different antisera, to compare their results.The research has been carried out with the aid of grants from the Medical Research Committee of the United Liverpool Hospitals under the Chairmanship of Lord Cohen of Birkenhead, the Nuffield Foundation and the Medical Research Council. Our thanks are due to our Senior Technician, Mr W. T. A. Donohoe, for his expert serological work. Finally we are grateful to our research assistant, Miss S. M. Manning, who has been invaluable in collecting most of the material and in carrying out all the secretarial wor
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