A conjugate of formula (A): the dotted lines indicate the optional presence of a double bond between C1 and C2 or C2 and C3; R2 is independently selected from H, OH, =0, =CH2, CN, R, OR, =CH-RD, =C(RD)2, 0-S02-R, C02R and COR, and optionally further selected from halo or dihalo; where RD is independently selected from R, C02R, COR, CHO, C02H, and halo; R6 and R9 are independently selected from H, R, OH, OR, SH, SR, NH2, NHR, NRR', NO2, Me3Sn and halo; R7 is independently selected from H, R, OH, OR, SH, SR, NH2, NHR, NRR', N02, Me3Sn and halo; Y is selected from a single bond, and a group of formulae A1 or A2: where N shows where the group binds to the N10 of the PBD moiety; RL1 and RL2 are independently selected from H and methyl, or together with the carbon atom to which they are bound form a cyclopropylene group; CBA represents a cell binding agent; Q is independently selected from O, S and NH; R11 is either H, or R or, where Q is O, S03M, where M is a metal cation; R and R' are each independently selected from optionally substituted C1-12alkyl, C3-20 heterocyclyl and C5-20aryl groups, and optionally in relation to the group NRR', R and R' together with the nitrogen atom to which they are attached form an optionally substituted 4-, 5-; 6- or 7-metfibered heteracyciie ring; wherein R12 R16, R19 and R17 are as defined for R2, R6, R9 and R7 respectively: wherein R" is a C3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, e.g. O, S, N(H}; NMe and / or aromatic rings, e.g. benzene or pyridine, which rings are optional y substituted; X and X' are independently selected from O, S and N(H).