This paper reports studies investigating the effects over time of age and social class grouping in infant day care on the psycho-social development of low socioeconomic status children (primarily black). Adult-child and child-child interactions were assessed in four different types of day care groupings: homogeneous x hetergeneous, age x social class (i.e., 2 x 2) groups. Observations were made of social interactions and task related actions of 80 2- and 3-year-olds during free play and organized work to provide data concerning effects of grouping on caregiving, intellectual stimulation, socialization and peer interaction. (This part of the study was carried out between pre- and posttesting of trust, persistence and intellectual achievement.) Findings indicate that heterogeneous grouping in infant day care settings facilitates the development of trust, assertiveness and autonomous achievement striving in the children enrolled. Heterogeneous grouping also had positive effects on caregivers' behaviors. Results and data analyses are presented. More than 200 pages of tables and appendices are included. (SB)