This study explores the power of intonation to convey meaningful information about the communicative intent of the speaker in speech addressed to preverbal infants and in speech addressed to adults. Natural samples of infant- and adult-directed speech were recorded from 5 mothers of 12-month-old infants, in 5 standardized interactional contexts: Attention-bid, Approval, Prohibition, Comfort, and Game/Telephone. 25 infant-directed and 25 adult-directed vocalizations were electronically filtered to eliminate linguistic content. The content-filtered speech stimuli were presented to 80 adult subjects: 40 experienced parents and 40 students inexperienced with infants. The subjects' task was to identify the communicative intent of the speaker using only prosodic information, given a 5-alternative forced choice. Listeners were able to use intonation to identify the speaker's intent with significantly higher accuracy in infant-directed speech than in adult-directed speech. These findings suggest that the prosodic patterns of speech to infants are more informative than those of adult-adult speech, and may provide the infant with reliable cues to the communicative intent of the speaker. The interpretation of these results proposed here is that the relation of prosodic form to communicative function is made uniquely salient in the melodies of mothers' speech, and that these characteristic prosodic patterns are potentially meaningful to the preverbal infant.