Abstract(#br)Effect of humus fractions on the toxicity of 2-methyl-4-chlorphenoxyacetic acid (MCPA) was examined in water cultures. Humus fractions and different MCPA quantities were added to Richter's nutrient solution used for maize ( Zea mays L., cv. Kočovská raná) cultivation. The results show that MCPA and humus substances in the applied concentrations intensively influence the maize root system, especially the rhizodermis and apical meristem cells. MCPA reduces ion absorption, induces phosphorus excretion from root, reduces tissue hydratation and relatively quickly kills these cells, at the end stage and/or in the highest concentration. Humus fractions have an unambiguously positive effect on these processes, they increase the cell vitality. When humus substances are applied, the... MCPA effect does not appear in potassium and nitrogen absorption, the inhibitory effect is maintained in phosphorus absorption and in tissue hydratation even with the most effective fraction—with fulvo acids. But all fractions increased the cell vitality of sorption tissue and meristems in the presence of MCPA.