Reference levels of some biochemical and haematological variables were determined for mute swans (Cygnus olor Gmelin). Immaturity was identified as a factor which broadened the references ranges. Blood variables of six acutely lead-poisoned swans were determined and compared with the reference distribution. Protoporphyrin IX, cholesterol and two serum enzymes, lactate dehydrogenase and aspartate aminotransferase, were increased; concentrations of haemoglobin and mean cell haemoglobin concentration indicated hypochromic anaemia in birds with acute lead poisoning. The interpretation and diagnostic value of reference values is discussed. © 1988, Taylor & Francis Group, LLC. All rights reserved.