In a family, in the first instance investigated in a linkage study, blood grouping showed MNSs distributions which could not be expected by inheritance of complexes MS, Ms, NS, and Ns, as generally assumed. (father: MS/Ns, mother: MS/Ns or Ms/NS, first child: MS/NS, second child: Ns/Ns). Since illegitimacy of one or both of the children could be excluded practically with certainty, only a mutation at the MN or Ss locus, or a crossing-over between the MN and the Ss locus can explain the children's genotypes, the crossing-over being the most probably right interpretation of MNSs findings in this family.