The p50 subunit of NF-kappa B is derived from the amino terminus of a 105 kilodalton precursor. The p105 carboxyl terminus, which contains ankyrin-like repeats, a feature of I kappa B molecules, regulates the cytoplasmic retention of p105 and inhibits DNA binding by the precursor. Here, we describe an I kappa B protein identical to the carboxyl-terminal region of p105. Probes spanning the COOH terminus but not the rel homology domain of p105 hybridize to a distinct 2.6-kilobase mRNA expressed in a wide range of murine tissues. The nucleotide sequence of complementary DNA clones for this transcript, in vitro translation, and immune precipitation of metabolically labeled cell lysates establish that it encodes a 70 kilodalton protein that corresponds to the COOH-terminal 607 amino acids of p105. p70 suppresses p65 and p75c-rel mediated transactivation of reporter genes under the control of NF-kappa B elements and in vitro can prevent DNA binding of p50 and p75c-rel homodimers to NF-kappa B sites. The ability of p70 to stably associate with p49 and p65 in vitro, but not inhibit DNA binding by these proteins, suggests that the specific inhibitory properties of this I kappa B may reflect its relative affinity for different rel targets. p70 phosphorylated by protein kinase A fails to inhibit DNA binding by p50 or the c-rel protein, and sequencing of radiolabeled p70 tryptic phosphopeptides establishes that protein kinase A phosphorylates serine residue 576 of p70. This finding suggests that the inhibitory activity of p70 can be regulated by signaling via the adenylate cyclase pathway.