Our interest in the male accessory glands (MAGs) of Leptinotarsa decemlineata was raised recently by our finding that certain cells produce a secretory substance that is recognized by one of our monoclonal antibodies (MAC-18), developed for the immunohistochemical demonstration of peptidergic neurons in the brain. We undertook to isolate this substance, presumably a peptide, to find out more about its role in the post-mating physiology of the recipient of this peptide, the mated female. This paper describes the purification and chemical characterization of the immunoreactive peptide from 100 pairs of male accessory glands. The peptide was purified by two subsequent reversed-phase-HPLC runs, and fractions were analyzed on Western blots that were immunostained by MAC-18. This indicated the presence of an 8 kDa peptide in the MAG. Partial analysis of the N-terminal amino acids by automated Edman degradation revealed a sequence of 40 amino acid residues. To obtain the full amino acid sequence of this peptide, the technique of reverse transcriptase PCR (3'RACE)was used. A PCR product of 350 bp was obtained, which encoded the 3'-end of the mRNA. After cloning and sequencing, this product contained most of the genetic information of the MAG peptide. The PCR product was also used as a probe for screening a eDNA library constructed from mRNA extracted, from MAGs. The nucleotide sequence coding for the signal peptide was: elucidated by 5'RACE. The cDNA and 5'RACE clones were analyzed and sequenced. The sequence of the cDNA clone contained an insert of 411 bp, which agreed well with the mRNA size measured by Northern blotting. Translation of the DNA sequences confirmed the data from partial amino acid sequence analyses and also predicted the remainder of the amino acid sequence. The entire peptide, designated Led-MAGP, consists of 74 residues; its mass was calculated and confirmed by mass spectrometry at 7971 Da. The peptide contains seven imperfect hexa-repeats, and this hexa-repeat sequence shows remarkable similarity to the hexa-repeat section of the chicken prion protein. The physiological function of the peptide has yet to be determined, but the hexa-repeat motif has recently been identified as the signal that induces internalization of the prion protein by coated-pit mediated endocytosis. Possible implications for the control of reproductive activities in L. decemlineata are discussed.