This article evaluates the effect of a nationwide public policy designed to reduce discrimination in access to accommodation in the private rental housing market. We carried out a correspondence test with 3260 real estate agencies to assess the likelihood of discrimination based on ethnic origin. This test allowed us to identify about 10% of agencies likely to exhibit discriminatory behavior. These agencies were randomly divided into two groups: one group received a formal letter warning them that they had been monitored and reminding them of the legal sanctions to which they were exposing themselves and the other group received no communication. We then re-tested all these real estate agencies for two years and compared their responses. The results indicate that the warning message has a substantial and long-lasting effect on discrimination: it decreases discrimination in the treated group by 80% for at least two years. •We assess the impact of a formal warning to reduce housing discrimination.•The warning is reminder of the law sent by a French institution.•We evaluate the effect of the warning by crossing two experimental methods.•The warning message has a substantial and long-lasting effect on discrimination.