The aim of this paper is to assess the change of Polish numeral jeden ‘one’ into an indefinite marker in the view of the grammaticalization theory. Although Slavic languages are principally believed not to possess articles, certain usages of one (e.g., in Bulgarian and Macedonian) demonstrate the same features as the ones ascribed to the usages of indefinite articles in non-Slavic languages, such as English, German or Italian. Language contact of article-possessing languages is often claimed to enhance the grammaticalisation process of an indefinite article (Heine and Kuteva 2006). This type of grammaticalisation is said to follow five distinctive stages: (i) numeral, (ii) presentative marker, (iii) specific marker, (iv) non-specific marker and (v) generalized article (e.g., Givón 1981, Heine 1997). We assessed that in the case of Polish, the grammaticalisation stage is that of a specific marker, with some occasional uses leaning towards the non-specific marker stage. The conclusion was supported by the results of 53 native speakers’ judgments as well as the diagnostic tests based on relevant literature.