The ability to recognize individuals is essential to many aspects of social behaviour, such as the maintenance of stable social groups, parent-offspring or mate recognition, inbreeding avoidance and the modulation of competitive relationships. Odours are a primary mediator of individuality signals among many mammals. One source of odour complexity in rodents, and possibly in humans, resides in the highly polymorphic major histocompatibility complex (MHC). The olfactory acuity of mice and rats allows them to distinguish between the urinary odours of congenic strains differing only in single genes within the MHC, although the chemical mediators or odorants are unknown. However, rodent urine also contains a class of proteins, termed major urinary proteins (MUPs), that bind and release small volatile pheromones. We have shown that the combinatorial diversity of expression of MUPs among wild mice might be as great as for MHC, and at protein concentrations a million times higher. Here we show in wild house mice (Mus domesticus) that urinary MUPs play an important role in the individual recognition mechanism.