Infants of 18 and 24 months acquiring English were tested in a preferential looking task on their ability to detect ungrammaticalities caused by manipulating a single function word in sentences. Infants heard grammatical sentences in which the determiner "the" preceded a target noun, as well as three ungrammatical conditions in which "the" was either dropped, replaced by a nonsense function word ("el"), or replaced by an alternate English function word ("and"). Both the 18- and 24-month-old infants oriented faster and more accurately to a visual target following grammatical sentences. The results suggest that by 18 months of age, infants use their knowledge of determiners in sentence computation and in establishing reference.