FIELD : medicine. SUBSTANCE : invention relates to the field of medicine, in particular to new peptides possessing biocidal, in particular antibacterial, antifungal and antiviral properties and to preparations based on them. Proposed biocidal peptide of the general formula Y-Ile-Leu-Pro-X-Lys-X-Pro-X-X-Pro-X-Arg-Arg-NH2, where X = 4-nitrophenylalanine; 4-chlorophenylalanine; 4-methoxyphenylalanine; D-phenylalanine; 4-aminophenylalanine; 4-aminobenzoylphenylalanine; homophenylalanine; 4-tert-butylphenylalanine; 2-methylphenylalanine; 4-fluorophenylalanine; pentafluorophenylalanine or 2-trifluoromethylphenyl-alanine; Y = H, palmitoyl or aminodecanoyl and the preparation in the form of a gel, having biocidal properties, containing as active principle a peptide in the concentration of 0.001–0.1 wt. %. Inventive peptides exhibit biocidal properties against bacteria, including spore, mold fungi, as well as viruses. EFFECT : peptide-based gels can be used to treat bacterial, viral, and concomitant infectious diseases. 5 cl, 16 tbl, 9 ex