This longitudinal study examines the early stages of language acquisition in four first-born infants from birth to two years. Selected aspects of vocal-verbal behavior exhibited by the children and their parents were examined. Home and laboratory recordings, diaries, interviews, and developmental observation were principal data sources. Substudies concentrating on the first 24 weeks of life included analyses of (1) fundamental frequency characteristics of nondistress vocalizations; (2) the evolution of prototype conversations; and (3) the perceptual and acoustic attributes of infant vocalization. The children displayed a rich and varied sound repertoire which seems to parallel changing aspects of evolving sensory-motor behaviors. Parent child-directed language appears to have different functions depending on such factors as the child's developmental level and the parents' perception of abilities and preferences. Preliminary findings indicate that in-depth examination of small numbers of children in a variety of interactive and non-social settings (including the home) needs to begin within the first month of life so that a large body of data, available for repeated scrutiny, can be generated. (Author/MS)