SummaryFifty MZ and 51 same-sex DZ twins were studied for ABH salivary secretions and the amount of these substances measured in those with the same A1A2BO blood groups. Using the classical, manual inhibition test we verified: (a) Levels for the H antigen in A2and B persons similar to, but in O and A1lower than those obtained in other places; (b) Low averages for the A and B substances; (c) Mean intrapair differences for A and H, which were the double in DZ as in MZ twins, the estimated degree

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... f genetic determination being 0.86 and 0.69 respectively; in both cases theF-ratio was significant at the 0.01 level; (d) If one discrepant family is disregarded the correlation coefficient between O parents and DZ children for the H antigen (0.48) is of the order of magnitude expected under a simple additive model of polygenic inheritance.Tests were also made using a fully automated method. The intraclass correlation coefficients thus obtained in MZ and DZ were similar to those reached using the data obtained with the classical technique. The degree of concordance arrived at by the two methods was only fair, but this may have been due to extraneous factors related to the way the samples were handled.