The purpose of this paper was to examine whether a pediatrician in well child care could promote mother-child interaction in the infant's first 6 months of life, and whether this intervention could affect the infant's cognitive development. Thirty-two mothers and their healthy, first born infants were followed by one pediatrician at 2, 4, 8, 15 and 27 weeks of age. These dyads were randomly assigned to a control group, who received customary care, or to an intervention group, who also received guidance based on the infant's developmental status at each age. Prior to a 27-week visit, the mother-infant relationship was assessed by a person blind to the group assignment. At the same time, infant development was assessed with the Bayley Mental Scale of Infant Development and two of the Uzgiris-Hunt Ordinal Scales. No differences were found on the Bayley M.D.I. or the Object Permanence Scale, but experimental infants were more advanced than control infants on the Vocal Imitation Scale. The intervention group mothers were rated significantly higher than control group mothers on sensitivity, cooperation, appropriateness of interaction, and appropriateness of play. It was concluded that this intervention was effective with both the mother-infant relationship and infant development. (Author/MP)