A homogeneous peptide with m683 Da which inhibits HIV-1 integrase with IC50 3 x 10(-5) M was separated from aqueous extracts of marine worm Eunicidae sp. by multi-stage chromatography purification. The structure Asp-Leu-Hse-His-Ala-G1n was proposed for this peptide according to the amino acid analysis, automated amino acid Edman sequencing, TLC with the witness and homoserine MS/MS fragmentation. The proposed structure is the first example of natural peptide containing amino acid homoserine residue.