Soyoung Lee,1 Dongwoo Khang,2 Sang-Hyun Kim1 1CMRI, Department of Pharmacology, School of Medicine, Kyungpook National University, Daegu, Republic of Korea; 2Department of Molecular Medicine, Graduate School of Medicine, Gachon University, Incheon, Republic of Korea Background: From the various physiochemical material properties, the chemical functionalization order of single-walled carbon nanotubes (swCNTs) has not been considered as a critical factor for modulating immunological responses and toxicological aspects in drug delivery applications. Although most nanomaterials, including carbon nanotubes, are specifically accumulated in spleen, few studies have focused on spleen immunotoxicity. For this reason, this study demonstrated that the dispersity of swCNTs significantly influenced immunotoxicity in vitro and in vivo. Materials and methods: For cytotoxicity of swCNTs, MTT assay, reactive oxygen species production, superoxide dismutase activity, cellular uptake, and confocal microscopy were used in macrophages. In the in vivo study, female BALB/c mice were intravenously administered with 1 mg/kg/day of swCNTs for 2 weeks. The body weight, organ weight, hematological change, reverse-transcription polymerase chain reaction, and lymphocyte population were evaluated. Results: Different orders of chemical functionalization of swCNTs controlled immunotoxicity. In short, less-dispersed swCNTs caused cytotoxicity in macrophages and abnormalities in immune organs such as spleen, whereas highly dispersed swCNTs did not result in immunotoxicity. Conclusion: This study clarified that increasing carboxyl groups on swCNTs significantly mitigated immunotoxicity in vitro and in vivo. Our findings clarified the effective immunotoxicological factors of swCNTs by increasing dispersity of swCNTs and provided useful guidelines for the effective use of nanomaterials. Keywords: carbon nanotubes, immunotoxicity, dispersity, physicochemical property