Fig. 1. Pregnancy failure after exposure to whole urine and urine mixed with MHC class I peptides. MHC class I peptides function as individual identification signals in the circumstance of pregnancy block in mice. A significant pregnancy failure rate was observed when female C57BL/6 mice were mated with C57BL/6 male mice and then exposed to BALB/c male urine or the H-2^d haplotype peptide in C57BL/6 urine (*P<0.05). Pregnancy block in females exposed to familiar male urine or the H-2^b haplotype peptide was not significant. A significant effect was not observed when C57BL/6 female mice were mated with BALB/c male mice and then exposed to familiar urine. Female C57BL/6 mice mated with BALB/c male mice and exposed to BALB/c male urine supplemented with H-2^b haplotype peptide or C57BL/6 male urine showed a significant pregnancy failure rate (*P<0.05).