A number of recent studies have argued that bilingual children's language comprehension and production may be affected by cross-linguistic influence. The overall aim of this study was to investigate whether the ability to judge the grammaticality of a construction in one language is affected by knowledge of the corresponding construction in the other language. We investigated how English-Italian and Spanish-Italian bilingual children and monolingual peers judged the grammaticality of plural NPs in specific and generic contexts in English and in Italian. We also explored whether language of the community, age, and the typological relatedness of the bilinguals' two languages significantly affected their performance. While performance in English was overall poor, no significant differences existed between the English-Italian bilinguals and the monolinguals. In contrast, we found that knowledge of English affected the bilinguals' ability to discriminate between grammatical and ungrammatical sentences in Italian. The English-Italian bilinguals were significantly less accurate than both the monolinguals and the Spanish-Italian bilinguals in a task where they simply had to rely on the local definite article cue to reject ungrammatical bare plurals in generic contexts. Language of the community and age also played a significant role in children's accuracy.