We have molecularly cloned a cDNA encoding a new Rel-related protein in Xenopus laevis. The product is most homologous to mammalian p100-NFkappaB2. Furthermore, the putative protein kinase A-phosphorylation site (RRPS), which is found in most of the Rel family proteins and is replaced by KRKR in mammalian p100, is also replaced by KRKK in our clone, indicating that our cDNA most likely encodes the Xenopus p100 (Xp100). Like mammalian p52, a processed product of p100, Xp52 alone binds to the kappaB site but does not activate transcription, while the XRelB/Xp52 heterodimer activates transcription, which is inhibited by the carboxyl-terminal half of Xp100 (XIkappaBdelta). Xp100 transcripts are present at all stages of oocyte maturation and in all adult tissues examined. Xp100 transcripts decrease at the gastrula stage and resume their expression at the neurula stage, which is different from other Xenopus rel family. Xp100 is highly expressed in somitogenic mesoderm at the neurula stage, while in the gastrula and tailbud stages, Xp100 transcripts are not localized to restricted regions. These results suggest that Xp100 could be involved in the late-stage development of Xenopus laevis, especially in the maturation of somites.