A compound of the following formula la: where m is 0 to 2; n is 0 or 1; X stands for -C(=O)-NH 2 or -COOH, L a stands for a self-immolative linker, A 1 stands for a residue of one of the amino acids Gly, Pro, Ala, Val, Nva, Leu, Ile, Met, Phe, Tyr, Trp, Ser, Thr, Cys, Asn, Gln, Asp, Glu, Lys, Arg, citrulline or His, or one of the respective N-alkyl amino acids, A 2 stands for a residue of one of the amino acids D-Ala, D-Pro, D-Ser, D-Asn, D-Asp or D-His, R 2 stands for -H or C 1 -C 3 alkyl, or R 2 stands for the binding to the methylene group of the proline ring, in the event of A 2 standing for a residue of D-Pro, D stands for -D 1 -(L b) o - p, D 1 stands for a cytotoxic agent, LIG stands for a binder, which, after binding to a target molecule of a tumour cell, is internalised by the tumour cell and processed intracellularly, preferably lysosomally, L b stands for a linker, o and p are 0 or 1, in each case independently of one another, R stands for Z 1 -(C=O)q-, q is 0 or 1, Z 1 stands for a C 1-10 alkyl-, C 5-10 aryl- or C 6-10 aralkyl-, C 5-10 heteroalkyl-, C 1-10 alkyl-O-C 6-10 aryl-, C 5-10 heterocycloalkyl-, heteroaryl-, heteroaryl-alkyl-, C 5-10 heteroaryl-alkoxy-, C 1-10 alkoxy-, C 6-10 aryl-, C 1-10 alkyloxy-, C 6-10 aryloxy- or C 6-10 aralkoxy-, C 5-10 heteroalkoxy-, C 1-10 alkyl-O-C 6-10 aryloxy- or C 5-10 heterocyclo-alkoxy group, which can be substituted once or more by -NH 2,-C(=O), -NH-alkyl, -N(alkyl) 2, -NH-C(=O)-alkyl, -N(alkyl)-C(=O)-alkyl,-S(=O) 3 -H, -S(=O) 2 -NH 2, -S(=O) 2 -N(alkyl) 2, -COOH, -C(=O)NH 2, -C(=O)-N(Alkyl) 2 or -OH, or for -H or a group -(CH 2) 0-1 -O x -(CH 2 CH 2 O) v -R 1, x is 0 or 1 v is a number from 1 to 20, R 1 stands for -H, -alkyl, -CH 2 -COOH, -CH 2 -CH 2 -COOH or -CH 2 -CH 2 -NH 2, or R stands for LIG-(L c) r, LIG stands for a binder, which, after binding to a target molecule of a tumour cell, is internalised by the tumour cell and processed intracellularly, preferably lysosomally, L c stands for a linker; and r is 0 or 1. m is 0 to 2; n is 0 or 1; X stands for -C(=O)-NH 2 or -COOH, L a stands for a self-immolative linker, A 1 stands for a residue of one of the amino acids Gly, Pro, Ala, Val, Nva, Leu, Ile, Met, Phe, Tyr, Trp, Ser, Thr, Cys, Asn, Gln, Asp, Glu, Lys, Arg, citrulline or His, or one of the respective N-alkyl amino acids, A 2 stands for a residue of one of the amino acids D-Ala, D-Pro, D-Ser, D-Asn, D-Asp or D-His, R 2 stands for -H or C 1 -C 3 alkyl, or R 2 stands for the binding to the methylene group of the proline ring, in the event of A 2 standing for a residue of D-Pro, D stands for -D 1 -(L b) o - p, D 1 stands for a cytotoxic agent, LIG stands for a binder, which, after binding to a target molecule of a tumour cell, is internalised by the tumour cell and processed intracellularly, preferably lysosomally, L b stands for a linker, o and p are 0 or 1, in each case independently of one another, R stands for Z 1 -(C=O)q-, q is 0 or 1, Z 1 stands for a C 1-10 alkyl-, C 5-10 aryl- or C 6-10 aralkyl-, C 5-10 heteroalkyl-, C 1-10 alkyl-O-C 6-10 aryl-, C 5-10 heterocycloalkyl-, heteroaryl-, heteroaryl-alkyl-, C 5-10 heteroaryl-alkoxy-, C 1-10 alkoxy-, C 6-10 aryl-, C 1-10 alkyloxy-, C 6-10 aryloxy- or C 6-10 aralkoxy-, C 5-10 heteroalkoxy-, C 1-10 alkyl-O-C 6-10 aryloxy- or C 5-10 heterocyclo-alkoxy group, which can be substituted once or more by -NH 2,-C(=O), -NH-alkyl, -N(alkyl) 2, -NH-C(=O)-alkyl, -N(alkyl)-C(=O)-alkyl,-S(=O) 3 -H, -S(=O) 2 -NH 2, -S(=O) 2 -N(alkyl) 2, -COOH, -C(=O)NH 2, -C(=O)-N(Alkyl) 2 or -OH, or for -H or a group -(CH 2) 0-1 -O x -(CH 2 CH 2 O) v -R 1, x is 0 or 1 v is a number from 1 to 20, R 1 stands for -H, -alkyl, -CH 2 -COOH, -CH 2 -CH 2 -COOH or -CH 2 -CH 2 -NH 2, or R stands for LIG-(L c) r, LIG stands for a binder, which, after binding to a target molecule of a tumour cell, is internalised by the tumour cell and processed intracellularly, preferably lysosomally, L c stands for a linker; and r is 0 or 1.