In this study the social behaviour of young children with autism spectrum disorder (ASD) and their mothers is compared within two different dyads: a dyad consisting of a mother and her own child and a dyad consisting of a mother and an unfamiliar child. Mothers did not change the frequency of their social initiatives and responsiveness with an unfamiliar child, but they became less directive than with their own child. Children with ASD did not show significantly better social behaviour with their own mother than with an unfamiliar mother. The results suggest that the social behaviour of a child with autism is not significantly enhanced by the familiarity of the social partner, but rather by the partner's autism-adapted interaction style. Clinical implications of these findings have been discussed. (Contains 4 tables and 1 note.)