Microalgae of the genus Pseudo-nitzschia are present in almost all marine and estuarine ecosystems and are the basis of the food chain. Some species of the genus Pseudo-nitzschia produce neurotoxin, domoic acid (DA). Individuals who have consumed food containing DA develop disorder called amnesic shellfish poisoning. Oxidative stress is result of an imbalance in oxido-reduction reactions in the body and in humans is associated with the development of numerous diseases. The aim of this study was to explore on human blood cells oxidative stress as a mechanism of toxicity of metabolites of some Pseudo-nitzschia species. In the first step of the study three species of the genus Pseudo-nitzschie were grown: P. delicatissima (PN1), P. pseudodelicatissima (PN2) and P. calliantha (PN3), and the nutrient medium in which the microalgae grew was collected. In the next step blood samples from healthy voluntary donors were treated with collected medium as well as with sea and sea and media (that served as controls) for 4 and 24 hours. After treatment in blood samples glutathione (GSH) as marker of anti-oxidative defence, malondialdehyde (MDA) as marker of lipid peroxidation and protein carbonyls (PC) as marker of oxidative damage to proteins were determined spectrophotometrically. For statistical analysis Student t-test (Excel, MS Office) was used, and the significance level was set at p ≤ 0.05. In blood samples exposed for 4- and 24-hours to medium in which Pseudo-nitzschie PN1, PN2 and PN3 grew an increase in GSH level was observed. However, only in blood samples exposed for 24-hours to medium in which P. pseudodelicatissima (PN2) grew significantly higher GSH level in comparison to control is recorded (p ≤ 0.05). Exposure to medium in which Pseudo-nitzschie grew decreased MDA level in blood samples. After 24-hour exposure of blood cells to medium in which P. delicatissima (PN1) was grown, a significant decrease in MDA concentration compared to the control group was observed (p ≤ 0.05). 4- and 24-hour exposure of blood cells to medium in which PN1, PN2 and PN3 were grown led to a decrease in PC level, however, the decrease was not significant compared to control. The observed increase in GSH level in blood samples exposed to the medium in which Pseudo-nitzschie grew indicate that in blood cells activation of the defence against oxidative stress occurred. Decrease in MDA and PC level may be explained with increased GSH level that prevented oxidative damage to lipids and proteins. Therefore, it can be concluded that the tested Pseudo-nitzschie species into the nutrient medium secrete metabolite(s), which can induce oxidative stress in human blood cells.