Kin bias during social interaction among house mice (Mus musculus) is influenced by learned odour cues used by the animal when encountering and interacting with other individuals. In this experiment with wild mice, the source of these odour cues was investigated by cross-fostering single male pups between unrelated litters to create mixed-paternity litters. Observation of dyadic interactions involving juvenile or adult mice reared in mixed-paternity litters and unfamiliar animals reared in single-paternity litters showed a bias in social investigatory and passive contact behaviours towards (a) an individual's own kin or the kin of an unrelated foster-littermate and (b) the unrelated foster-littermate of a relative, compared with (c) an unfamiliar unrelated control. The functional significance of kin bias is discussed in the light of the role of odour cues in recognizing familiar social group members and the transfer of odours between individuals within groups.