Cylindrospermopsin (CYN) is a natural toxin synthesized by several freshwater cyanobacteria (blue-green algae) species. Their expansion in Central European surface waters has been observed in last decades. Potential identified effects of CYN on human health exposed mainly via contaminated drinking water or consuming contaminated food includes hepato-, geno-, cyto- and fetal toxicity. So far there is very little known about CYN effects on immune system. We investigated the effect of different concentrations of this toxin upon the human peripheral blood lymphocytes proliferation and observed significant inhibition induced after 24 h by 1 μg ml^-1 CYN. We believe this is the first report of CYN effect on this process in human lymphocytes. Authors will continue to study immunotoxicity of this chemical compound.