During paternity testing, we encountered the following constellation in the Jk system: the mother's phenotype was Jk(a-b+), while the son was typed as Jk(a+b-). The deduced genotype of the mother would have been Jkb Jkb, and each offspring should then express the Jk(b) antigen. Consequently, non-maternity would be deduced. Since no material was available for extended family studies or HLA typing, except for the DNA of the propositi, only RFLP analysis could bring clarification in this case. The application of four highly polymorphic single locus probes proved the maternity and hence the existence of a Jk-Null allele. We conclude that direct testing at the DNA level may help resolving cases where, by conventional parentage testing, conclusive results are unachievable because of putative 'Null' alleles.