The pseudofirst-order ozonization rate constant of whole bovine blood has been measured in comparison to that of free haemin. The free prosthetic group haemin (which has also the central iron atom in the oxidized form) shows k values in the range of 0.20-0.03 s(-1) while the haeme groups inside haemoglobin protein and contained in the whole blood sample show slightly lower k values, just in the range of 0.10-0.02 s(-1). It has been found that ozone even with whole blood reacts specifically with haemoglobin of the red cells because it is adsorbed selectively on the iron atoms of the haeme prosthetic groups of haemoglobin. The absorption implies the oxidation of the central iron atom of the haeme groups with formation of methaemoglobin followed by an oxidative fission of the haeme rings. The other blood components do not exert any significant protection to the reaction between ozone and haemoglobin, which appear extremely specific and selective like the reaction between CO or HCN and haemoglobin. By analogy with the behaviour of these other gases ozone may be classified as a blood poison. The results of this work are discussed in the frame of the risks connected to the ozonotherapy and autohaemotherapy involving the blood ozonization of human or animal subjects and the re-injection of ozonized blood into the bodies.