Fig. 3. Pregnancy failure after exposure to urine of juvenile or castrated male mice. MHC class I peptides and urine, in the absence of an androgen influence, are able to induce pregnancy failure. C57BL/6 females mated with C57BL/6 male mice and exposed to C57BL/6 castrated male urine supplemented with the H-2^b peptide failed to block pregnancy. However, a significant effect was observed when C57BL/6 females mated with C57BL/6 males were exposed to C57BL/6 juvenile urine supplemented with the H-2^d peptide. A significant effect was observed when C57BL/6 females mated with C57BL/6 males were exposed to urine from BALB/c castrated males. Exposure of C57BL/6 female mice mated with BALB/c males to urine of castrated or juvenile C57BL/6 males resulted in a significant pregnancy failure rate, whereas, exposure to urine of BALB/c castrated males failed to block pregnancy. A significant pregnancy failure rate was observed when C57BL/6 female mice were mated with BALB/c males and exposed to urine of C57BL/6 castrated males supplemented with the H-2^d peptide. A significant result was again observed when urine from BALB/c castrated males supplemented with the H-2^b peptide was applied to the C57BL/6 females mated with the BALB/c males.