To assess the underlying bases of the phenotypic traits by which lambs are recognized, ewes' responses to both dizygotic and monozygotic twins were observed. When tested several hours post-partum, ewes discriminated between their familiar lamb and its same-sex dizygotic twin that had been isolated at birth. The latter twins were nonetheless treated more positively than were unfamiliar alien lambs. A similar pattern of results was obtained for ewes tested with familiar and isolated monozygotic twins, and an unfamiliar alien lamb. To assess the possible influence of the differential pre-test treatments of familiar and isolated twins and the role of maternal chemical labels in offspring discrimination, the above experiments were replicated with the familiar lamb confined to a mesh cage that prevented maternal licking and nursing. Although isolated dizygotic twins were accepted to a lesser extent than their caged familiar twins, levels of maternal acceptance/rejection did not differ between the isolated twins and unfamiliar alien lambs. In contrast, isolated monozygotic twins were not treated differently than their caged familiar twins, but they were nonetheless distinguished from unfamiliar alien young. These latter data tentatively suggest that ewes may have discerned a greater resemblance between familiar and isolated MZ twins than between DZ twins of these two categories. It therefore appears that lambs' individually distinctive signatures reflect an interaction between genetically influenced processes and non-genetic factors (e.g. environmental or metabolic).