Neuroblastoma, the most frequent malignant tumour in neonates and young children, has an unusual clinical behaviour, age being the most important single factor. This could indicate that some growth-regulating mechanism in lost, or changed, during the first years of life. In search for possible growth-modifying factors, we identified a tetrapeptide, acetyl-Asp-Gln-Tyr-GlyNH2, in extracts of neuroblastoma tissue, in neuroblastoma cell cultures and in new-born pig brain tissue. The purified native peptide as well as a synthetic peptide with the same structure decreases in vitro growth of neuroblastoma cells at a restricted, low (picomolar) range. The structure of the peptide as well as its dose-response characteristics, indicates that it belongs to a group of endogenous growth-modifying oligopeptides that previously have been isolated from other organs and tissues. A possible role for the new peptide in clinical medicine is discussed.