A peptide with a bacterial-growth-stimulating activity was isolated from a bovine hemoglobin hydrolysate by reversed-phase high-performance liquid chromatography. Its primary structure and molecular mass. determined by amino acid analysis and fast-atom bombardment mass spectrometry, were identical to those of fragment 48-52 (Ser-Thr-Ala-Asp-Ala) of the beta chain of bovine hemoglobin, The microbiological tests in solid media demonstrated that this peptide exhibited a growth-stimulating activity on gram-negative bacteria.