A method of designing a panel of bacteriophages as a therapeutic composition against a bacterial infection, the method comprising: (a) providing two or more different bacteriophages, wherein each of said two or more different bacteriophages independently retards growth of a target bacterial species or strain; (b) combining at least two of said two or more different bacteriophages; (c) determining growth of the target bacterial species or strain in the presence of said combination of two or more different bacteriophages, wherein the target bacterial species or strain growth conditions are the same or equivalent in steps (a) and (c); (d) wherein, when said combination retards growth of the target bacterial species or strain less than the greatest growth retardation achieved independently by any one of said two or more different bacteriophages, the combination is initially rejected as a panel of bacteriophages; (e) providing at least one further different bacteriophage that independently retards growth of the target bacterial species or strain, (f) replacing at least one bacteriophage from a combination specified in part (d) with the at least one further different bacteriophage to form a further combination; (g) determining growth of the target bacterial species or strain in the presence of said further combination; (h) wherein, when said further combination retards growth of the target bacterial species or strain at least equal to the greatest growth retardation achieved independently by any one of said different or further different bacteriophages, the combination is accepted as a panel of bacteriophages; and (i) wherein, when said combination retards growth of the target bacterial species or strain less than the greatest growth retardation achieved independently by any one of said different or further different bacteriophages, the combination is further rejected as a panel of bacteriophages. (a) providing two or more different bacteriophages, wherein each of said two or more different bacteriophages independently retards growth of a target bacterial species or strain; (b) combining at least two of said two or more different bacteriophages; (c) determining growth of the target bacterial species or strain in the presence of said combination of two or more different bacteriophages, wherein the target bacterial species or strain growth conditions are the same or equivalent in steps (a) and (c); (d) wherein, when said combination retards growth of the target bacterial species or strain less than the greatest growth retardation achieved independently by any one of said two or more different bacteriophages, the combination is initially rejected as a panel of bacteriophages; (e) providing at least one further different bacteriophage that independently retards growth of the target bacterial species or strain, (f) replacing at least one bacteriophage from a combination specified in part (d) with the at least one further different bacteriophage to form a further combination; (g) determining growth of the target bacterial species or strain in the presence of said further combination; (h) wherein, when said further combination retards growth of the target bacterial species or strain at least equal to the greatest growth retardation achieved independently by any one of said different or further different bacteriophages, the combination is accepted as a panel of bacteriophages; and (i) wherein, when said combination retards growth of the target bacterial species or strain less than the greatest growth retardation achieved independently by any one of said different or further different bacteriophages, the combination is further rejected as a panel of bacteriophages.