In 1977 a nonapeptide, called delta-sleep-inducing peptide (DSIP) was characterized in rabbit cerebral venous blood plasma during thalamic stimulation to induce sleep. Evidence for the existence of DSIP in the central nervous system and in numerous peripheral organs of various mammalian species has been obtained using immunochemical techniques. Later findings have revealed the existence of large forms of DSIP-like immunoreactivity. We decided to investigate the molecular identity of such large forms of DSIP-like immunoreactivity by direct isolation. We have purified and characterized using amino acid analysis, sequencing, mass spectrometry and radioimmunoassay a 77-residue peptide, denoted DIP (DSIP-immunoreactive peptide), from an acid extract of porcine brain. DIP is recognized by an antiserum raised against synthetic rabbit DSIP. The amino acid sequence of DIP, however, is not related to that of DSIP, but it contains a putative leucine-zipper motif, a proline/glutamic-acid-rich domain, three potential phosphorylation sites and exhibits an acetylated N-terminus. The N-terminal but not the C-terminal part of the newly isolated peptide shares clear homology with the sequence of a protein induced by transforming growth factor β1 and other growth factors in mouse osteoblastic cells. DIP is also structurally similar to a baculoviral protein p10. The function of DIP remains unclear but its involvement in transcriptional regulation is probable.