Rapid acquisition of linguistic categories or constructions is sometimes regarded as evidence of innate knowledge. In this paper, we examine Polish children's early understanding of an idiosyncratic, language-specific construction involving the instrumental case--which could not be due to innate knowledge. Thirty Polish-speaking children aged 2; 6 and 3; 2 participated in a elicited production experiment with novel verbs that were demonstrated as taking nouns in the instrumental case as patients. Children heard the verbs in sentences with either masculine or feminine nouns (which take different endings in the instrumental case), and were tested with new nouns of the same and of the opposite gender. In both age groups, a substantial majority of children succeeded in generalizing from one gendered form of the instrumental case to the other (especially to the masculine), thus indicating that they have some kind of abstract understanding of the instrumental case in this construction. This relatively early abstract knowledge of an idiosyncratic construction casts doubt on the view that early acquisition requires innate linguistic knowledge.