1. The effects of nitrates and nitrites on growth, erythrocytic count, liver and kidney functions, humoral and cell mediated immune responses in cockerels were investigated. 2. Sodium nitrate (4.2 g/kg diet) and sodium nitrite (1.7 g/kg) retarded growth, caused methaemoglobinaemia and changes in erythrocytic count, serum concentrations of glutamic-pyruvic transaminase, creatinine and urea. 3. Cockerels given nitrate or nitrite in the food had reduced haemagglutination responses after injection of sheep erythrocytes and a reduced delayed hypersensitivity reaction to purified protein derivative tuberculin following sensitisation to Bacille Calmette Guerin. 4. Nitrates and nitrites are environmental pollutants present in food and water and they may contribute to the aetiology of liver and kidney diseases and problems related to failure of immunity in domestic fowls.