Uptake of poly-dispersed acid functionalized single-walled carbon nanotubes (AF-SWCNTs) in resting and activated B and T cells was studied by using fluorescence tagged AF-SWCNTs. Activated B and T cells (activated by LPS and staphylococcal enterotoxin B respectively), internalized substantially higher amounts of AF-SWCNTs as compared to the resting cells. The uptake of AF-SWCNTs was significantly elevated in both dividing and non-dividing T and B cells activated cultures but the increase was significantly more for dividing cells. Confocal microscopy indicated poor uptake of AF-SWCNTs by resting B and T cells whereas much higher uptake was seen in activated B and T cells. Effect of AF-SWCNTs on the kinetics of accumulation of live B and T cells in activated and proliferating cultures indicated that AFSWCNTs exerted significantly higher cytotoxic effect on activated B and T cells as compared to resting control cells. Targeting of AFSWCNTs specifically to activated cells in a mixture of resting and activated B and T cells was also demonstrated. Uptake of AF-SWCNTs was significantly lower at 4C as compared to 37C indicating that active uptake mechanisms were involved in AF-SWCNT uptake. Testing the effect of several known inhibitors of different pathways utilized by cells to internalize particles (chlorpromazine hydrochloride, cytochalasin D, wortmannin and filipin), indicated that these inhibitors had minimal effect on the AF-SWCNT uptake in resting B and T cells but significant effects were observed when activated B and T cells were used. Overall our results show that activated B and T lymphocytes internalize substantially more AF-SWCNTs as compared to resting cells and open up the possibility of specific targeting of activated lymphocytes to abrogate these cell populations in vivo