This paper briefly reviews studies of infant-peer and infant-mother interaction, and describes a study which compares the effects of mothers and peers on toddler play. Twenty-six toddlers (13 males and 13 females) aged 21 to 24 months were observed in two social situations each: playing with his/her mother, and playing with a familiar peer (after exit of the mother). Infant interactions with play objects in each of the two social situations were coded for the level of exploratory behavior and the degree of involvement of the child. In addition, observations were coded for each child under all of the four experimental conditions: mother and infant, infant alone, infant, mother and peer; and infant and peer alone. Results indicated significant sex differences: males exhibited a greater frequency of play activity with peers while females most frequently played with their mothers. Also, males showed a greater frequency of high level play (exploratory and creative combined) with peers, while females again indicated a greater level of high frequency play with their mothers. Hence, both mothers and peers are the best facilitators of high level and sustained play, depending on the gender of the child. (CM)