Carbon black nanoparticles (CB-NPs) have been synthesized from liquid benzene by a solution plasma method at room temperature and atmospheric pressure. The morphological observation by scanning electron microscopy revealed the agglomeration of aggregated fine particles. The synthesized CB-NPs were predominantly amorphous as confirmed by X-ray diffraction. The in vitro cytotoxicity of CB-NPs on the human lung fibroblast (MRC-5) cell line was assessed by the 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and systematically compared with those of two types of commercial carbon blacks (i.e., Vulcan XC-72 and Ketjenblack EC-600JD). Cell viabilities were studied at different concentrations of 32.5, 65, 125, and 250 mu g/mL. It was found that the CB-NPs derived from solution plasma exhibited a lower cytotoxicity on the MRC-5 cells than the other two comparative carbon blacks. The viability of MRC-5 cells exposed to CB-NPs remained higher than 90% even at a high concentration of 250 mu g/mL. This result preliminarily confirmed the biosafety and potential use of CB-NPs in the field of biological applications. (c) 2018 The Japan Society of Applied Physics