Drawing from an affect-induction model of laughter (Bachorowski & Owren, 2001; Owren & Bachorowski, 2002), we propose that "antiphonal" laughter--that is, laughter that occurs during or immediately after a social partner's laugh--is a behavioural manifestation of a conditioned positive emotional response to another individual's laugh acoustics. To test hypotheses concerning the occurrence of antiphonal laughter, participants (n = 148) were tested as part of either same- or mixed-sex friend or stranger dyads, and were audiorecorded while they played brief games intended to facilitate laugh production. An index of antiphonal laughter for each dyad was derived using Yule's Q. Significantly more antiphonal laughter was produced in friend than in stranger dyads, and females in mixed-sex dyads produced more antiphonal laughter than did their male partners. Antiphonal laughter may therefore reflect a mutually positive stance between social partners, and function to reinforce shared positive affective experiences.