We performed the micronucleus test to determine the level of biocompatibility of pristine multi-walled carbon nanotubes (MWNTs) and MWNTs functionalized with nylon-6, (referred to as the nylon-6/ MWNTs nanohybrid), when they interact with human lymphocytes in a cell culture medium. A comparative genotoxic analysis demonstrated a better degree of biocompatibility of nylon-6/MWNT with human lymphocytes, as compared to pristine MWNTs, for the concentration range of 10-60 microg/ml. An evidence was found that pristine MWNTs act as clastogenic agents and possibly as aneuploidogenic agents, increasing the frequency of genotoxic bioindicators. On the other hand, nylon-6/ MWNTs nanohybrid were observed to induce cell death via apoptosis, which could be attributed to residual impurities of epsilon-caprolactam.