AbstractNodularin (NOD) is a cyclic peptide released by bloom‐forming toxic cyanobacteria Nodularia spumigena commonly occurring in brackish waters throughout the world. Although its hepatotoxic effects are well known, other negative effects of NOD have not yet been completely elucidated. The present study aims were to evaluate and compare the cytotoxic and immunotoxic effects of the toxin on primary leukocytes (from head kidney [HK]) and stable fish leukocytes (carp leucocyte cell line [CLC] cells). The cells were incubated with the cyanotoxin at concentrations of 0.001, 0.01, 0.05, or 0.1 μg/ml. After 24 h of exposure, the concentrations ≥0.05 μg/ml of toxin resulted in cytotoxicity in the primary cells, while in CLC cells, the toxic effect was obtained only with the highest concentration. Similarly, depending on the concentration, exposure to NOD causes a significant inhibition of chemotaxis of the phagocytic abilities of primary leukocytes and a significant reduction in the proliferation of lymphocytes isolated from the HKs. Moreover, CLC cells and HK leukocytes incubated with this toxin at all the mentioned concentrations showed an increased production of reactive oxygen and nitrogen species. NOD also evidently influenced the expression of genes of cytokine TNF‐α and IL‐10 and, to a minor extent, IL‐1β and TGF‐β. Notably, the observed changes in the mRNA levels of cytokines in NOD‐exposed cells were evident, but not clearly dose‐dependent. Interestingly, NOD did not affect the production and release of IL‐1β of the CLC cells. This study provides evidence that NOD may exert cytotoxicity and immune‐toxicity effects depending on cell type and toxin concentration.