Macrophages are tightly associated with inflammatory diseases as well as carcinogenesis, and therefore represent promising targets for drug delivery and gene transfer. We have recently established a novel protein delivery system based on the binary C2 toxin of Clostridium botulinum and streptavidin, allowing the uptake of exogenous biotinylated molecules into mammalian cells. Here, we applied this C2-streptavidin delivery system to macrophages and other leukocytes. First, the effect of wild-type C2 toxin on different leukocyte cell lines was tested, indicating no differences in sensitivity. Next, the uptake and stability of the engineered C2-streptavidin was analyzed in macrophages and Jurkat T-cells, showing internalization into the cytosol of both cell types with similar kinetics. The transporter did not exhibit any cytotoxic effect and did not interfere with phagocytosis in primary human macrophages. The C2-streptavidin system promoted specific uptake of biotinylated fluorophores into the cytosol of macrophages as revealed by confocal microscopy. In addition, flow cytometry analysis showed a significantly enhanced uptake of biotinylated fluorescent tracers in Jurkat leukemia cells mediated by the C2-streptavidin transporter. Our results demonstrate that C2-streptavidin is a functional delivery system for transport of biotinylated molecules into macrophages and other leukocytes without compromising cell viability and intrinsic functions such as phagocytosis.