A conjugate of formula I: wherein Ab is a modified antibody having at least one free conjugation site on each heavy chain R 2 is of formula IIIa, formula IIIb or formula IIIc: where A is a C 5-7 aryl group, and either (i) Q 1 is a single bond, and Q 2 is selected from a single bond and -Z-(CH 2) n -, where Z is selected from a single bond, O, S and NH and n is from 1 to 3; or (ii) Q 1 is -CH=CH-, and Q 2 is a single bond; where; R C1, R C2 and R C3 are independently selected from H and unsubstituted C 1-2 alkyl; where Q is selected from O-R L1, S-R L1 and NR N -R L1, and R N is selected from H, methyl and ethyl X is selected from the group comprising: O-R L1, S-R L1, CO 2 -R L1, CO-R L1, NH-C(=O)-R L1, NHNH-R L1, CONHNH-R L1,, NR N R L1, wherein R N is selected from the group comprising H and C 1-4 alkyl; R L1 is a linker for connection to the antibody (Ab); R 2' is selected from the same groups as R 2 and is linked to the same antibody; R 6 and R 9 are independently selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; where R and R' are independently selected from optionally substituted C 1-12 alkyl, C 3-20 heterocyclyl and C 5-20 aryl groups; R 7 is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, e.g. O, S, NR N2 (where R N2 is H or C 1-4 alkyl), and/or aromatic rings, e.g. benzene or pyridine; Y and Y' are selected from O, S, or NH; R 10 and R 11 either together form a double bond between the nitrogen and carbon atoms to which they are bound or; R 10 is H and R 11 is selected from OH, OR A and SO z M; R 30 and R 31 either together form a double bond between the nitrogen and carbon atoms to which they are bound or; R 30 is H and R 31 is selected from OH, OR A and SO Z M; R 6', R 7' and R 9' are selected from the same groups as R 6, R 7 and R 9 respectively. Ab is a modified antibody having at least one free conjugation site on each heavy chain R 2 is of formula IIIa, formula IIIb or formula IIIc: where A is a C 5-7 aryl group, and either (i) Q 1 is a single bond, and Q 2 is selected from a single bond and -Z-(CH 2) n -, where Z is selected from a single bond, O, S and NH and n is from 1 to 3; or (ii) Q 1 is -CH=CH-, and Q 2 is a single bond; where; R C1, R C2 and R C3 are independently selected from H and unsubstituted C 1-2 alkyl; where Q is selected from O-R L1, S-R L1 and NR N -R L1, and R N is selected from H, methyl and ethyl X is selected from the group comprising: O-R L1, S-R L1, CO 2 -R L1, CO-R L1, NH-C(=O)-R L1, NHNH-R L1, CONHNH-R L1,, NR N R L1, wherein R N is selected from the group comprising H and C 1-4 alkyl; R L1 is a linker for connection to the antibody (Ab); R 2' is selected from the same groups as R 2 and is linked to the same antibody; R 6 and R 9 are independently selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; where R and R' are independently selected from optionally substituted C 1-12 alkyl, C 3-20 heterocyclyl and C 5-20 aryl groups; R 7 is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, e.g. O, S, NR N2 (where R N2 is H or C 1-4 alkyl), and/or aromatic rings, e.g. benzene or pyridine; Y and Y' are selected from O, S, or NH; R 10 and R 11 either together form a double bond between the nitrogen and carbon atoms to which they are bound or; R 10 is H and R 11 is selected from OH, OR A and SO z M; R 30 and R 31 either together form a double bond between the nitrogen and carbon atoms to which they are bound or; R 30 is H and R 31 is selected from OH, OR A and SO Z M; R 6', R 7' and R 9' are selected from the same groups as R 6, R 7 and R 9 respectively. (i) Q 1 is a single bond, and Q 2 is selected from a single bond and -Z-(CH 2) n -, where Z is selected from a single bond, O, S and NH and n is from 1 to 3; or (ii) Q 1 is -CH=CH-, and Q 2 is a single bond; R C1, R C2 and R C3 are independently selected from H and unsubstituted C 1-2 alkyl; where Q is selected from O-R L1, S-R L1 and NR N -R L1, and R N is selected from H, methyl and ethyl X is selected from the group comprising: O-R L1, S-R L1, CO 2 -R L1, CO-R L1, NH-C(=O)-R L1, NHNH-R L1, CONHNH-R L1,, NR N R L1, wherein R N is selected from the group comprising H and C 1-4 alkyl; R L1 is a linker for connection to the antibody (Ab); R 2' is selected from the same groups as R 2 and is linked to the same antibody; R 6 and R 9 are independently selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; where R and R' are independently selected from optionally substituted C 1-12 alkyl, C 3-20 heterocyclyl and C 5-20 aryl groups; R 7 is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, e.g. O, S, NR N2 (where R N2 is H or C 1-4 alkyl), and/or aromatic rings, e.g. benzene or pyridine; Y and Y' are selected from O, S, or NH; R 10 and R 11 either together form a double bond between the nitrogen and carbon atoms to which they are bound or; R 10 is H and R 11 is selected from OH, OR A and SO z M; R 30 and R 31 either together form a double bond between the nitrogen and carbon atoms to which they are bound or; R 30 is H and R 31 is selected from OH, OR A and SO Z M; R 6', R 7' and R 9' are selected from the same groups as R 6, R 7 and R 9 respectively.