This study examined whether pitch patterns of prelinguistic vocalizations could discriminate between social vocalizations, uttered apparently with the intention to communicate, and "private" speech, related to solitary activities as an expression of "thinking". Four healthy ten month old English-speaking infants (2 boys and 2 girls) were simultaneously video- and audiorecorded in their homes under two conditions: (A) when the infant was playing with mother, and (B) when the infant was alone. One hundred and fifty-six vocalizations were classified as "communicative", if accompanied by non-vocal communicative behaviour, or as "investigative", if accompanied by explorative activities. For the acoustic analysis, an automatic PITCH PATTERN RECOGNITION (PPR) software system was developed. The PPR-system could distinguish "communicative" from "investigative" vocalizations with an overall accuracy of 91.67%. These findings confirm that prelinguistic vocalizations might serve both as means of purposeful communication and as a tool of thought. These are the functions later assumed by language. (Contains 1 footnote.)