Two-month-old infants (N = 29) participated in face-to-face interactions with their mothers and with strangers. The contingent responsiveness for smiles and vocalizations, while attending to the partner, was assessed for each partner in both interactions. For smiles and for vocalizations, infants were less responsive to the stranger relative to the mother when the stranger's contingent responsiveness was either more contingent or less contingent than that of the mother. Results are supportive of the hypothesis that young infants develop sensitivities to levels of social contingency present in their maternal interactions, which influence their responsiveness to others. (Contains 4 tables, 2 figures, and 1 footnote.)