Male house mice produce large quantities of major urinary proteins (MUPs), which bind and transport volatile pheromones to urinary scent marks. It has been suggested that MUP isoform profiles are highly polymorphic and provide a unique signature that mediates individual and kin recognition (barcode hypothesis). This hypothesis assumes that MUP profiles show high inter-individual variability and high intra-individual consistency, and we conducted the first study to test these assumptions. We analyzed urinary MUP profiles of 66 wild-caught house mice (Mus musculus musculus) from 8 populations using isoelectric focusing. We found that MUP profiles were not individually unique, and the variation depended on the type of isoform. Most proteins (the major isoforms) were surprisingly monomorphic, but the minor ones were variable and therefore candidates for individual barcodes. To test this hypothesis, we examined changes in 58 mice over time, and found that individual MUP profiles were dynamic and the minor isoforms showed the most fluctuation. Thus, most MUP isoforms were monomorphic, and though we found minor isoforms that were variable among individuals, they showed dynamic changes over time within mice. Nevertheless, we found more similarities in MUP profiles between siblings than non-siblings. Our findings challenge the hypothesis that urinary MUPs contain individual signatures, though it is feasible that they mediate kin recognition.