The biocompatibility of functionalized carbon nanohorns (f-CNH) was analyzed regarding primary phagocytic cells. Fluorescein isothiocyanate-labeled (FITC) f-CNHs were incubated with primary peritoneal mouse macrophages for different periods of time to analyze whether macrophages can sense and internalize f-CNHs. It was found using confocal microscopy that f-CNHs were not detected in all the cells after 24 hours of incubation. The investigation revealed that f-CNHs can be obtained by a direct addition to pristine CNHs of a diamine moiety that can be further modified through a fluorescent probe, resulting in biocompatible f-CNHs. The results show that the delivery of biologically active agents using f-CNHs that internalize into cells and promote contemporarily the activation of the immune system, is a very encouraging concept.