In conversations between adults, attributions about a speaker's intentions are partly determined by vocal intonation. Similarly, 3-month-olds have been found to produce intonational vocal sounds during social interactions. The present experiment investigated whether infant vocalizations differing in intonational quality resulted in differential attributions of communicative intent by adults. Seventy-seven undergraduates viewed videotapes of 24 three-month- old infants who interacted vocally with an experimenter. Twelve of the infants produced intonational (syllabic) sounds and the other 12 infants produced sounds consisting of uniform pitch (vocalic). The infants were presented in random order, and gender and attractiveness were controlled (counterbalanced) across conditions of infant vocal quality. The adults completed rating scales designed to assess their attributions of intentionality and their perceptions of social favourability toward each of the infants. The results indicated that infants who produced the intonational syllabic sounds were rated as more intentionally communicative and socially favourable than those who produced vocalic sounds (p < 0.001). A series of confirmatory factor analyses confirmed that the rating scales did measure two different (but related) factors (intentionality and social favourability). These results are discussed in terms of the possible influence of attributions of intentionality on adults' behaviour and on the infant's development of communicative competence.