The effect of nitrogen-doped multiwall carbon nanotubes (CNx) on the proliferation of NIH-3T3 murine fibroblasts is presented. CNTs were dispersed in distillated water and incubated with mammalian cells in order to evaluate their toxicity. Also, the influence of factors such as dosage (7 and 70 µg/mL), exposure time (24 to 96 h), and the exposure route (before and after cell liftoff) on the cell proliferation was evaluated. When the CNxwere simultaneously incubated with the cells, the control culture reached a maximum cell concentration of 1.3 × 105 ± 3.4 × 104cells per well at 96 h, whereas cultures with 7 µg/mL reached a concentration of 2.6 × 104 ± 5.3 × 103cells. In the case of 70 µg/mL of CNxmost of the cells were dead. The CNxthat were added 24 h after cell dissociation showed that live cells decreased, with a cell concentration of 9.6 × 104 ± 9 × 103for 7 µg/mL and 5.5 × 104 ± 9.5 × 103for 70 µg/mL, in contrast to control cultures with 1.1 × 106 ± 1.5 × 104. The results showed that the CNxhad cytotoxic effects depending on the concentration and exposure route.