Preschool and many older children often have difficulty understanding who carries out the complement action (e.g., to go in sentences such as Mary promised John to go; this is so, even though they easily understand this information in sentences such as John told Mary to go. C. Chomsky (1969) proposed that children's errors arise from the overgeneral application of a purely structural Minimal Distance Principle. Maratsos (1974), however, hypothesized that children err by overgeneralizing a different principle which he called the Semantic Role Principle. According to this principle, the Goal-Recipient of the spoken message, not the closest noun phrase, is understood as the person who carries out the act referred to by the infinitival complement. The two studies reported here were designed to determine which of these principles children use. Preschool children were taught to understand a specially designed novel construction. The children then acted out sentences containing related but novel uses of the construction such that they would respond differentially according to which interpretive principle they used. The results strongly favored the Semantic Role Principle, rather than the MDP described by Chomsky (1969), or a related, more complex MDP described by Rosenbaum (1967). It is further discussed how children's formulation of the Semantic Role Principle, rather than the MDP, might arise from their prior analyses of related constructions, such as the imperative construction, rather than following from an innate preference, as suggested in Maratsos (1974).