The presence of cyanobacterial toxins in water and algae pose a health hazard for animals and humans, due to their tumour-promoting activity and carcinogen effects. The use of simple, rapid and reliable tools for routine analysis is becoming a necessity. With this purpose, our group has developed two electrochemical immunosensors for the detection of microcystin-LR (MC-LR) based on the affinity between this cyanotoxin and the corresponding monoclonal and polyclonal antibodies. A competitive direct enzyme-linked immunosorbent assays (ELISAs) was designed and, after validation of the approach on microtiter wells, screen-printed graphite electrodes were used as supports. Colorimetry was used to optimise the experimental parameters and to compare the performance of monoclonal and polyclonal antibodies. Afterwards, electrochemical measurements were performed at -200 mV (versus Ag/AgCl) using 5-methyl-phenazinium methyl sulfate (MPMS) as mediator for horseradish peroxidase (HRP), the enzymatic label of the competitor. The IC(50) values were 0.10 and 1.73 microgL(-1) for MAb and PAb, respectively. Whereas Mab provided higher sensitivities, the reproducibility was better when using PAb. The developed amperometric immunosensors were applied to the analysis of cyanobacterial samples from the Tarn River (Midi-Pyrénées, France) and the presence of MC was confirmed by the colorimetric protein phosphatase inhibition (PPI) assay and high performance liquid chromatography (HPLC). The limits of detection attained from the calibration curves and the results obtained for the real samples demonstrate the potential use of the immunosensors as screening tools for routine use in the assessment of water quality and the control of toxins in algae.