This study examined the social labels which are first used by infants, social differentiation on the basis of labeling behavior, and overgeneralization of social labels. Subjects were 81 infants from 9 to 36 months of age. The 9- to 24-month-olds were shown slides of themselves, their mothers, their fathers, and unfamiliar children, babies, and adults. The mother pointed to each slide and asked "Who is that?" The 30- and 36-month-olds were shown the same classes of persons but saw pictures mounted in a photograph album. The experimenter pointed to each picture and asked "Who is that?" Results indicated that infants were more likely to label their father's than their mother's picture and that they did so earlier and more accurately. The data also suggested that: (1) the child uses a word to represent several perceptual features of social objects; and (2) overgeneralization may occur when there are not enough labels available for all of the child's perceptual categories. Since the children in this study overgeneralized labels in an age- and sex-appropriate manner, their social labels appear to be a reflection of the way in which they categorize the social world. (JMB)