In 1984 we identified and characterized a growth-inhibiting pentapeptide, pyroGlu-Glu-Asp-Ser-GlyOH, [EPP] from mouse epidermis. Later, other pyroGlu N-terminal oligopeptides have been isolated and characterized from liver and mouse intestine. The three pyroGlu-terminal mitosis inhibitory peptides are structurally similar and have several biological properties in common. A number of questions remain, however, to be answered, e.g., a) Are the peptides part of larger molecules; b) Do they bind to specific receptors on the target cells; c) How are they related to other growth-modulating factors, and c) Are they coded for by genes that are related to known growth regulating proto-oncogenes, especially to growth suppressing genes. To search for soluble parts of possible receptors, or carrier molecules, in water extracts of mouse epidermis we have used affinity columns coated with EPP with either the N-terminal end or the carboxy-end free. Both types of column bind a 70 kD protein. The protein bound to the column with a free N-terminal end splits into two small components under reducing conditions. To look for larger molecules of which EPP could be a fragment, we have used western blotting techniques and a polyclonal rabbit antiserum against EPP. Preliminary experiments have indicated that two different molecules bind to the antiserum.