Conceptus mortality is greatest in mammals during the peri-implantation period, a time when conceptuses appose and attach to the uterine surface epithelium while releasing proinflammatory molecules. Interleukin 1 beta (IL1B), a master proinflammatory cytokine, is released by the primate, rodent, and pig blastocyst during the peri-implantation period and is believed to be essential for establishment of pregnancy. The gene encoding IL1B has duplicated in the pig, resulting in a novel gene. Preliminary observations indicate that the novel IL1B is specifically expressed by pig conceptuses during the peri-implantation period. To verify this, IL1B was cloned from mRNA isolated from Day 12 pig conceptuses and compared with IL1B cloned from mRNA isolated from pig peripheral blood leukocytes (PBLs). The pig conceptuses, but not the PBLs, expressed a novel IL1B, referred to here as interleukin 1 beta 2 (IL1B2). Porcine endometrium was treated with recombinant porcine interleukin 1 beta 1 (IL1B1), the prototypical cytokine, and IL1B2 proteins. Immunohistochemistry and real-time RT-PCR were used to measure activation of nuclear factor-kappa B (NFKB) and NFKB-regulated transcripts, respectively, within the endometrium. Both IL1B1 and IL1B2 activated NFKB in the uterine luminal epithelium within 4 h. The NFKB activation and related gene expression, however, were lower in endometrium treated with IL1B2, suggesting that the conceptus-derived cytokine may have reduced activity within the uterus. In conclusion, the peri-implantation pig conceptus expresses a novel IL1B that can activate NFKB within the uterine surface epithelium, likely creating a proinflammatory microenvironment during establishment of pregnancy in the pig.