The existence of rare cells with blood group A or B phenotype among the red cells of AB heterozygotes is a well-known phenomenon. However, its origin remains unclear due to methodological problems. A direct quantitation of non-B and non-A erythrocytes in A1B donors revealed minor populations of only-A and only-B cells, respectively, both in a frequency of 10(-3). Null cells comprise, at the most, a fraction of about 5 X 10(5). In order to discriminate between somatic crossingover (SCO) and gene inactivation as the underlying mechanism, three individuals were selected who were double heterozygotes for the blood group (AB) and the linked locus of adenylate kinase (AK-2-1). Separation of cells with A or B phenotypes did not result in cosegregation of the AK isoenzymes. Thus, the variant blood group phenotypes represent the normal frequency of allelic silence, not the product of SCO. This sets a background variant level for the estimation of somatic recombination in blood cells from normal donors. Determination of variant phenotypes in a Bloom's syndrome patient, heterozygous for AB, gave a frequency six times higher than the value of normals. It is suggested that this elevated figure might indicate the frequency of SCO, which is known to be higher in Bloom's syndrome.