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In this study, the impacts of lead toxicity on Oreochromisniloticus were investigated. Additionally, the potential ameliorative effects of the Spirulina algae Arthrospira platensis were evaluated. The median lethal concentration (LC 50 ) of PbNO 3 was determined to be 143.3 mg/l for O. niloticus weighing 42 ± 2.5 g. O. niloticus were exposed to 10 % of the estimated PbNO 3 LC 50 for 12 weeks. The cumulative mortality rate (CMR) increased with exposure time. The results of assays for red blood cells (RBCs), haemoglobin (Hb), packed cell volume (PCV), mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), and mean corpuscular haemoglobin concentration (MCHC) indicated that the exposed O. niloticus suffered from anaemia. The levels of liver enzymes, namely, aspartate transaminase (AST) and alanine transaminase (ALT), as well as metallothionein)MT(revealed deterioration of hepatic tissue. The activity of the antioxidant enzymes glutathione peroxidase (GPx) as well as catalase (CAT) was stimulated in the hepatic tissue of O. niloticus exposed to PbNO 3 and in those treated with A. platensis. Based on the results of serum bactericidal activity (SBA) and oxidative burst activity (OBA) assays as well as challenge tests with Aeromonas hydrophila, it was clear that supplementation with 5 or 10 g/kg A. platensis significantly enhanced the fish immune status and decreased the mortality rate (MR). However, these effects were reduced by PbNO 3 exposure with no differences in MR percentage. Therefore, it was clear that O. niloticus reared in lead nitrate-polluted water were immunosuppressed, while diet supplementation with A. platensis could ameliorate such impacts., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)