Hybridisation of an human Y chromosome clone (pDP 1007) with Eco RI restricted DNA from male and female humpback whales revealed restriction-fragment patterns similar to those previously reported in other placental mammals. The sex-specificity of these restriction fragments in humpback whales was verified using DNA from necropsy samples of 11 individuals from the Southern Gulf of Maine. Necropsy samples from a fin (Balaenoptera physalus), a sei (B.