The present invention relates to amino acid sequences that are directed against interleukin 23 (IL-23). The amino acid sequences of the present invention comprise two NANOBODY® molecules against IL-23 and one NANOBODY® molecule against serum albumin, linked by two linkers. In particular, the invention relates to the amino acid sequences of SEQ ID NO: 2 and SEQ ID NO: 3 (listed in Table 1 and FIG.