We study the causal impact of revealing pro-unionism during the recruitment stage on hiring chances. To this end, we conduct a randomised field experiment in the Belgian labour market. When matched with employer and sector data, the experimentally gathered data enable us to test the heterogeneity of discrimination against pro-union applicants by the union density in the sector and the size of the firm. We find that disclosure of pro-unionism affects hiring chances in a negative way and that – in line with our expectations based on the literature – this negative impact is stronger in highly unionised sectors.