This paper, part of an evaluation of the early intervention project at the Frank Porter Graham Child Development Center, deals primarily with results of the Bayley Mental Development Index (MDI). The MDI was administered to experimental and control infants from lower socioeconomic background at 6, 12 and 18 months of age. Results (discussed in detail) indicate that the positive effects of early intervention can clearly be seen by the middle of the second year of life. Disadvantaged infants, as a group, tested within the normal limits at 6 and at 12 months. The experimental group maintained this level at 18 months, but the control group showed a sharp decline in the second year. Also discussed were features in the test performances which helped account for the differences in tested intellectual levels between the disadvantaged children in the intervention program and those who were not. (MS)