A pair of blood-group-chimeric twins, TS and MR and their family have been investigated with the single-locus-probe DNA technique for restriction fragment length polymorphism in five variable-numbers-of-tandem-repeat systems. An admixture of DNA from the other twin could be demonstrated in both twins, leading to a possible false-genotype determination in at least one system. Chimerism is a potential pitfall in DNA investigations with single-locus probes in forensic genetics.