The Rel/NF-kappa B family of proteins includes several interacting cellular transcription factors and the v-Rel oncoprotein of the avian Rev-T retrovirus. We report the isolation of a chicken cDNA for the NF-kappa B p52 precursor protein p100. Full-length p100 only weakly binds DNA in vitro; removal of the ankyrin-like repeats generates C-terminally truncated p100 proteins (like p52) that have an increased ability to bind an oligonucleotide containing a kappa B site. In addition, we show that chicken p100 is identical to a protein previously designated p115, which is found in a complex with v-Rel in transformed chicken spleen cells. Furthermore, p100 and v-Rel can form a complex when synthesized in vitro. Using cDNAs for chicken NF-kappa B p105, NF-kappa B p100, c-Rel, and v-Rel, we show that one of the complexes in v-Rel-transformed spleen cells can be reconstituted in vitro.