The aim of the present study, set in the Polish-English context, is to discuss to what extent, if any, L1 competence (semantic, phraseological and grammatical) is influenced by the language contact, understood here as the (advanced) knowledge of L2, i.e. English. The research is based on the questionnaire consisting of 23 short excerpts taken from newspaper articles or texts published on the Internet, adapted by the author of the article. The texts in the questionnaire contain various kinds of borrowings and calques from English, such as semantic loans, loan translations (new phrases, collocations or idiomatic expressions) and grammatical constructions modelled on English. The questionnaire was given to two groups of informants: the main group (i.e. the bilingual one), consisting of 4th year students of English at the University of Silesia enrolled in the teaching programme, and the control group, consisting of students of various fields (excluding English or Polish studies) who declared limited knowledge of English. The respondents from both groups were given the questionnaire and their task was to read the sentences carefully and then to decide whether some words or phrases seemed incorrect or unnatural in a given context. The study has corroborated the view that language contact has some repercussions at the individual level: the knowledge of L2 may and does influence L1 competence. Not all the areas (semantic, phraseological and grammatical), however, are affected to an equal extent.