Fig. 2. Pregnancy failure caused by MHC peptides only. Prototypical representative ligands for H-2^b and H-2^d haplotypes induce pregnancy failure when administered without urinary constituents. C57BL/6 female mice mated with C57BL/6 male mice and exposed to the active form of the MHC peptide specific for the H-2^d haplotype resulted in a significant pregnancy failure rate. The inactive H-2^d peptide failed to block pregnancy. C57BL/6 female mice mated with BALB/c male mice and exposed to the H-2^b haplotype peptide also showed a significant pregnancy failure rate. The inactive form of the H-2^b peptide failed to block pregnancy. Alt C57BL/6 and Alt. BALB/c refers to the control peptides, which had their characteristic anchoring residues changed to alanines, i.e. AAPDARETA and SAFPEITHA, respectively.