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[Display omitted] • Nanoplastics result in significant hepatic immune toxicity in zebrafish larvae. • Nanoplastic accumulation contributes to the ROS generation and the immunotoxicity. • Nanoplastics activate steroid hormone biosynthesis. • Immune transgenic zebrafish is sensitive to the immunotoxicity of nanoplastics. Nanoplastics in aquatic environments may induce adverse immunotoxicity effects in fish. However, there is insufficient evidence on the visible immunotoxicity endpoints in the larval stages of fish. The liver plays an important role in systemic and local innate immunity in the fish. In this study, the hepatic inflammatory effects of polystyrene (PS) nanoplastic particles (NPs: 100 and 50 nm) and micron PS particles on transgenic zebrafish (Danio rerio) larvae were estimated using fluorescent-labeled neutrophils, macrophages, and liver-type inflammatory binding protein (fabp10a). Particles with smaller size induced higher aggregations of neutrophils and apoptosis of macrophages in the abdomen of the larvae, corresponding to greater hepatic inflammation in the larvae. NPs increased the expression of fabp10a in the larval livers in a dose- and size-dependent manner. PS particles of 50 nm at a concentration of 0.1 mg·L−1 increased the expression of fabp10a in the larval liver by 21.90% (P < 0.05). The plausible mechanisms of these effects depend on their distribution and the generation of reactive oxygen species in the larvae. Metabonomic analysis revealed that the metabolic pathways of catabolic processes, amino acids, and purines were highly promoted by NPs, compared to micron PS particles. NPs also activate steroid hormone biosynthesis in zebrafish larvae, which may lead to the occurrence of immune-related diseases. For the first time, the liver was identified as the target organ for the immunotoxicity effects of NPs in the larval stage of fish. [ABSTRACT FROM AUTHOR]