Extracellular mold metabolites, which are toxic or have other adverse biological effects on humans and animals, form a group of mycotoxins. These compounds are of very different chemical structures and contaminate cereals and other foods, especially in the tropics because high temperatures and humidity favor mold formation and growth. People are often exposed to mycotoxins by consuming food, but in some cases it can be inhaled, they can pass through the skin or the exposure is parenteral. Fumonisins are mycotoxins most commonly produced by F. verticillioides and F. proliferatum. Since its discovery, fumonisins have been considered mycotoxins mainly produced by Fusarium species. The B-group of fumonisins (FB1, FB2, and FB3) is often detected in maize and maize-based foods. Today, at least 28 different forms of fumonisin are known, and most of them are designated as A-, B-, C-, and P-series. The most significant are B-series fumonisins, of which fumonisin B1 (FB1) and fumonisin B2 (FB2) are of almost equal toxicity. In nature, FB1 occurs 3 times more frequently than FB2. The aim of this study is to evaluate the cytotoxic concentrations at which cell survival is reduced by 50% (IC50) of fumonisin B1 and fumonisin B2 on two different cell lines: human lung adenocarcinoma cells, A549, and human hepatocellular carcinoma cells, HepG2. The determination of IC50 is used to select the subcytotoxic concentrations of these two types of fumonisins to be used in the study of the cytotoxic effect of FB1 and FB2 in combination with other mycotoxins occurring in dust of humid, air and food. To examine the cytotoxic effect of FB1 and FB2, A549 and HepG2 cells were treated with toxins at concentrations of 10 to 500 μM within 24 h. The viability was determined by MTS proliferation assay (490 nm). MTS proliferation assay was applied to human A549 and HepG2 cells. FB1 is poorly cytotoxic to A549 cells, that means that it significantly reduces cell viability only after administration at the highest concentrations (400 μM and 500 μM) relative to control. FB2 promotes proliferation of A549 cells by increasing concentration. HepG2 cells are more sensitive to the action of both fumonisins with FB2 having a slightly stronger cytotoxic effect than FB1; FB2 significantly reduces cell viability at a concentration of 150 μM and FB1 at twice as high. For both fumonisins it was not possible to determine IC50 in A549 and HepG2 cells, because the fumonisins, administered in the concentration range of 10 μM to 500 μM, didn`t cause a viability decrease of more than 50% of the cells.