Minisatellite DNA probes that detect highly polymorphic regions of the human genome were used to examine DNA from twelve sets of newborn twins. In the seven cases where the twins were known to be monozygotic or dizygotic, from sex observation or placental examination, the DNA result agreed with these findings. In the other five twin pairs and in two sets of triplets DNA analysis allowed rapid determination of zygosity. Minisatellite DNA probes provide a single genetic test that should allow positive determination of zygosity in all cases of multiple pregnancy.