This paper presents a study of the effects of "typical" infant day care on children's behavior and development. The variables studied include center versus home programs, length of time in day care, age of child and sex of child. Subjects for the study were 114 boys and girls (ranging in age from 12 to 36 months) enrolled in either a day care center or one of eight day care homes. All nine programs were described as providing a safe, pleasant and stimulating environment rather than as being oriented toward enrichment or education. Subjects were rated by an adult responsible for their care every six months over a period of 1 1/2 years on a battery of 12 developmental and temperamental tasks. Some of the results include the following: (1) center care children, especially those who had been in the program from four to six quarters, were superior in motor development and on one cognitive variable to children in home programs; (2) home care children were found to be more compliant and likeable than center children; (3) for all children, attention-curiosity, perseverance and activity scores increased as a function of time in day care; (4) no scores declined as a function of length of time in day care; (5) no sex differences were found. Possible interpretations of these and other results are discussed. (BD)