A novel tetrapeptide modulating the oviduct contractions was characterized from egg mass of Sepia officinalis. After two purification steps by rpHPLC, an apparent pure fraction containing the biological activity was submitted to MALDI-TOF analysis. The mass spectrum revealed 6 peaks of m/z 293, 505, 596, 613, 728, and 745. The tissue peptide mapping performed in LC-MS demonstrated the occurrence of the mit 505 peptide in the follicles, the full-grown oocytes, and in the eggs. This peptide was also recovered in the seawater after the incubation of full grown oocytes or eggs, demonstrating a release in the genital tract and in the environment. Edman degradation gave the following sequence: Ileu-Leu-Met-Glu. The synthetic peptide applied to the whole genital tract triggered a cyclisation of the contractions at 10(-14) M. ILME appeared to be a chemical messenger released by the oocytes and the eggs, and was able to exert both paracrine and pheromonal activity.