A conjugate of formula (A): where 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, SO3M, where M is a metal cation.
