► We studied parent–infant interaction in six-month-old infants with a sibling with ASD. ► Play interactions were rated for global parent, infant and dyadic characteristics. ► At-risk sibs were less lively than controls, possibly reflecting a trait difference. ► Parents of at-risk sib infants tended to have a more directive interactive style. ► Research must explore if disrupted interaction may amplify infant social atypicality. Recent models of the early emergence of autism spectrum disorder (ASD) propose an interaction between risk susceptibility and the infant's social environment, resulting in a progressively atypical developmental trajectory. The infant's early social environmental experience consists mostly of interaction with caregivers, yet there has been little systematic study of early parent–infant interaction in infants at risk of ASD. This study examined the global characteristics of parent–infant interaction in 6- to 10-month-old infants with an older sibling diagnosed with ASD (at-risk sibs), in comparison with a group of infants with no family history of ASD (low-risk sibs). As part of the British Autism Study of Infant Siblings (BASIS), 6-min videotaped unstructured play interactions of mother–infant dyads (45 at-risk sibs and 47 low-risk sibs) were rated on global aspects of parent–infant interaction, blind to participant information. Differences in global characteristics of interaction were observed in both infant and parent contributions in the at-risk group compared to low-risk controls. In analyses adjusted for age and developmental level, at-risk sib infants were less lively, and their parents showed higher directiveness, and lower sensitive responding (as a trend after adjustment). Level of infant liveliness was independent of other interactive behaviour. Consistent with reports in previous literature in older children with autism and in other neurodevelopmental disorders, our findings may suggest that infants at genetic risk are exposed to a more directive interactive style relatively early in infancy. We discuss possible explanations for these findings and implications for further developmental study and intervention.