In a study of ways of preventing mental retardation in the population of a city slum, surveys were conducted. The major finding was that the variable of maternal intelligence was the best single predictor of the level and character of intellectual development in the offspring. The survey also showed that the lower the maternal IQ, the greater the probability of offspring scoring low on intelligence tests. A longitudinal, prospective investigation was made of the determinants of the kind of retardation which perpetuates itself from parent to child in the "slum-dwelling" family. Using maternal IQ as a basis for selection of a group of newborns, comprehensive intervention in the social environment was undertaken with 40 newborns of mentally retarded mothers (IQ less than 70). The objective of this intervention was to displace all of the presumed negative factors in the social environment of the infant. The age range of the children was about one year, with the oldest being about four years. Results showed that the experimental children at 42 months surpassed the control group on the order of 33 IQ points, and had a learning capacity surpassing that anticipated. It is hoped that the data may prove that it is possible to prevent mental retardation in children reared by parents who are both poor and limited in ability. Nine figures illustrate the study. (DB)