This study investigates L2 effects on L1 grammar in foreign language learners. As partof a cross-sectional study, 25 Polish native speakers learning English and German, and16 Polish monolingual speakers participated in an acceptability judgment test in Polish The test involved two grammatical phenomena: anaphoric object pronouns which lie at thesyntax-pragmatics interface, and verb placement in wh-questions, which is a propertyof narrow syntax. The analysis shows that multilingual learners accepted overt anaphoricobject pronouns in a sentence-internal position significantly more frequently than monolingual speakers from the control group. Object pronouns in the native language seem to bean element open to the influence of a foreign language, in contrast to linguistic propertieswhich are solely syntactic. This study thus confirms that interface phenomena are moreprone to cross-linguistic influence than purely syntactic features, but it also extends thisthesis to include L2 effect on L1