Nanodiamonds (NDs) are unique, multifunctional nanoparticle with increasing clinical applications. The nitrogen vacancy (NV)-centers within the diamond lattice endows nanodiamonds with fluorescent properties. NDs are considered nontoxic, biocompatible and photostable compared to other nanoparticles which makes it suitable for in vivo applications. In the present study, we evaluated the uptake of carboxylated fluorescent nanodiamonds (cFNDs) by different subpopulations of spleen in vitro and by resting and activated spleen B and T cells. We also compared this data with the uptake of fluorescence tagged acid functionalized single walled carbon nanotubes (FAFSWCNTs). We found that there was poor uptake/binding of cFNDs by resting B and T cells, however, we could see the internalization of cFNDs by activated B and T Cells. Also, the percentage of cells positive for cFNDs increased significantly in activated B and T cells but the increase was significantly lower than that seen for the uptake of FAF-SWCNTs. The higher internalization of cFNDs in activated B and T cells suggests the possibility of use of cFNDs for imaging of activated B and T cells in vivo.