This study explored the inter-relationships among daycare experience, temperament, and preschoolers' social behaviors in a novel peer play laboratory setting. Sixty-two pairs of same-sex 5-year-olds played in a laboratory playroom for 20 minutes. Their behaviors were rated on prosocial and aggression scales. Parents completed questionnaires on daycare experience, temperament, and behavior problems. Path analyses showed that both daycare experience and temperament were important for predicting level of prosocial behaviors displayed during the peer play interactions. Sex of the child significantly predicted both aggressive and prosocial behaviors, with boys being both more aggressive and more prosocial. Children who experienced little or no daycare were more likely to behave prosocially, suggesting that daycare may actually inhibit socialization for some children. Thus, the variables of temperament and daycare experience are important to consider in formulating a fuller understanding of preschoolers' social behaviors.