The mothers of four pigtail (Macaca nemestrina) infants living in a group were removed for 4 weeks. All infants reacted initially with agitation. Three of the four infants then became severely depressed. The depression lasted about a week and was strikingly similar to the "anaclitic depression" of human infants who lost their mothers. When they were reunited, all four dyads showed a marked and prolonged intensification of the mother-infant relationship.