A cyclosporin derivative which is a compound of formula (I), or a pharmaceutically acceptable salt thereof, for use in the treatment or prevention of a viral infection: wherein: A represents (F(II)) or (F(III)) B represents methyl or ethyl, R2 represents ethyl or isopropyl, R4 represents &mdashCH2CH(CH3)CH3, &mdashCH2CH(CH3)CH2CH3, &mdashCH(CH3)CH3 or &mdashCH(CH3)CH2CH3, and either (a) one of R1 and R1* represents hydrogen and the other represents methyl, and R3 represents &mdashL3-G3, or (b) one of R1 and R1* represents hydrogen and the other represents and R3 represents H, wherein L1 and L3 represent a direct bond, a C1-C6 alkylene group or a C2-C6 alkenylene group and G1 and G3 represent a hydrogen atom, a &mdashCOOR&prime group, or a phenyl moiety which is unsubstituted or substituted by one, two or three substituents selected from a halogen atom, a &mdashCOOR&prime group, a &mdashCONR&primeR&Prime group, a hydroxyl group, a C1-C6 alkyl group and a C1-C6 alkoxy group, wherein R&prime and R&Prime are the same or different and represent hydrogen or a C1-C6 alkyl group, provided that (a) G1 does not represent a hydrogen atom when Li represents a direct bond or methylene, and (b) G3 does not represent a hydrogen atom when L3 represents a direct bond or methylene.
