6-month-old infants selected on irritability shortly after birth and their mothers were randomly assigned to 2 intervention and 2 control groups to test the hypothesis that enhancing maternal sensitive responsiveness will improve quality of mother-infant interaction, infant exploration, and attachment. The intervention lasted 3 months and ended when the child was 9 months of age. When infants were 9 months of age, intervention group mothers were significantly more responsive, stimulating, visually attentive, and controlling of their infant's behavior than control group mothers. Intervention infants had higher scores than control infants on sociability, self-soothing, and exploration, and they cried less. Quality of exploration also improved, with intervention infants engaged in cognitively sophisticated kinds of exploration more than control infants. At 12 months of age, significantly more intervention group dyads were securely attached than control group dyads.