Reproduction among related individuals is generally maladaptive. Inbreeding imposes significant costs on individual reproductive success, and can decrease population fitness. Theory predicts that polyandrous females can avoid inbreeding by exploiting paternity-biasing mechanisms that enable differential sperm 'use'. Evidence of sperm selection is difficult to demonstrate because patterns of non-random paternity can be generated by a variety of different mechanisms. Here, using in vitro fertilisation in mice, we provide evidence of sperm selection at the gametic level. We mixed the sperm of sibling and non-sibling males, and observed a fertilisation bias towards the sperm of non-sibling males. The number of motile sperm and sperm swimming performance did not differ between competitors among the replicate assays. Therefore, our result can only be ascribed to egg-driven sperm selection against related sperm. We conclude that the expression or secretion of gametic proteins could provide the molecular basis for this mechanism of cryptic female choice. & Sons Ltd/CNRS.