Mixtures of multi-walled carbon nanotubes (MWCNTs) with natural humic acids (Leonardite humic acid, LHA) or humic acid-like polycondensates (HALP) were evaluated, for the first time, about their potential genotoxic and cytotoxic effects in cultured human lymphocytes. The genotoxic evaluation of the tested materials, either separately or in combination, for the detection of micronuclei (MN) in the cytoplasm of interphase cells, was performed using the cytokinesis block micronucleus (CBMN) assay. A comparative analysis of the genotoxicity and cytotoxicity reveals that in the tested concentrations, the [MWCNTs + LHA] mixture is more genotoxic and slightly more cytotoxic than the [MWCNTs + HALP] mixture. MN induction observed in human lymphocytes demonstrates that humic substances enhance the genotoxic effects of MWCNTs. In addition, the present data highlight a – so far unforeseen – potential genotoxic effect as the result of both clastogenic and aneugenic actions of the particular mixtures on human lymphocytes.