objective: Protein therapeutics have potential to elicit immune responses resulting in undesirable anti-drug antibodies (ADA) that might affect product efficacy and patient safety, and should be assessed in animals before applying the treatment to humans. In this paper, we aim to assess the immunogenicity and toxicokinetics of the mono-PEGylated recombinant human interleukin-11 (rhIL-11), a novel protein therapeutic for the treatment of chemotherapy-induced thrombocytopenia, in repeated administration to cynomolgus monkeys. methods: Enzyme-linked immunosorbent assay (ELISA) methods were developed to measure ADA responses and plasma PEGylated IL-11 (PEG-IL11) concentration in monkeys. Assay parameters of immunogenicity and toxicokinetics methods were evaluated during validation in accordance with regulatory guidelines. We also employed cell-based assays to test the neutralizing activity of ADA provoked in monkeys. results: The results showed that weak immunogenicity occurred in some monkeys after receiving repeated dose of 0.1-0.3 mg/kg by subcutaneous administration and disappeared after the recovery period. More pronounced immunogenicity occurred at high dose of 0.9 mg/kg, with a higher positive rate and titer, and some ADAs had neutralizing activity, but it can be greatly reduced after recovery. Such ADAs generated in monkeys may be accounted for the plasma toxicokinetics changes of PEG-IL11 and a minor reduction in systemic exposure. conclusions: These methods have been successfully applied to immunogenicity and toxicokinetic studies of PEG-IL11 in repeated dose toxicity following subcutaneous administration to monkeys, and could be successfully used in clinical trials after some modifications.