This paper describes the purposes and procedures of a longitudinal study designed to: (1) relate mother-infant interaction patterns to infant age, sex, and social class; (2) relate mother-infant interaction patterns to infant sensory-motor development; and (3) to examine the relationship between infant sensory-motor development and infant sex and social class. Subjects are infants from 5 weeks to 42 weeks of age, from middle and lower class families. Data were collected by mother-infant observation and developmental testing of cognitive and perceptual-motor development. The specific rationale for the study and proposed approaches for analysis of the data are discussed. (Author/BRT)