Functionalized carbon nanohorns (f-CNH) are uptaken by macrophages, without affecting cell viability. f-CNH induce the production of reactive oxygen species and of pro-inflammatory cytokines. The level of inflammation, although moderate, should be taken into consideration when using f-CNH for drug delivery. However, it could be exploited as an intrinsic f-CNH adjuvant function for biomedical applications requiring some activation of the immune system.