Carbon nanotubes (CNTs) are emerging asinnovative tools in nanobiotechnology. However, theirtoxic effects on environment and health have become anissue of great concern. The effect of single-walled carbonnanotubes (SWCNTs) on primary immune cells in vitrowas studied in this paper. The results indicated thatSWCNTs (25 and 50 mg/mL) could promote the proliferation of spleen cells. However, they had no significanteffect on the proliferation of spleen cells at concentrationsof 1 and 10 mg/mL. They also had no effect on T-lymphocyte proliferation stimulated by concanavalinA (ConA) atlower concentrations. Moreover, they turned to inhibit Tlymphocyte proliferation at higher concentrations. It wasfound that SWCNTs inhibited the B-lymphocyte proliferation stimulated by lipopolysaccharides (LPS) at concentrations of 1, 10, 25 and 50 mg/mL. What is more, theysignificantly decreased the Natural Killer (NK) cell activity compared with the control group at all tested concentrations. The results suggest that SWCNTs have possiblynegative effects on immune cells in vitro.