Blood group B-gene-specified α-galactosyltransferase was determined in serums and erythrocyte membranes of several related persons with B(m) phenotypes. The level of this enzyme in serums was similar to, but in erythrocyte membranes much lower than that found in normal B and AB persons.

Pedigree studies and serological characterization of three cis AB families are presented. Serums of these persons exhibit strongly reduced activities of A genespecified α-N-acetylgalactosaminyltransferase and only trace activities of B gene-dependent α-galactosyltransferase.