The problems posed by transfusion of homologous blood have led to the development of substances able to replace the gas transporting properties of blood. Perfluorocarbons (PFCs) emulsions and modified haemoglobin (Hb) solutions have been developed for this goal and are now tested in clinical assays. PFCs are synthetic fluorinated hydrocarbons, capable of dissolving large quantities of oxygen (O2; without binding) at high inspired concentrations of O2, and of delivering this O2 to the tissues. They are administered as emulsions containing particles with a diameter of approximately 0.2 micron, capable of entering the microcirculation. They are eliminated unchanged by the lungs within several days. Fluosol-DA 20% was the first PFC emulsion used in clinical practice. Currently, Oxygent, a second generation PFC emulsion, is being evaluated in clinical studies. The PFCs are not blood substitutes, but rather a means to ensure tissue oxygenation during extreme haemodilution. Solutions of free Hb do not have the antigenic characteristics of the blood groups, and do not require compatibility testing. They are fully saturated with O2 at ambient FiO2. The Hbs used are derived from either human or bovine sources, or via recombinant DNA technology. In order to maintain satisfactory intravascular half-life and O2 affinity, the Hb molecules are modified by adding internal crosslinks, by polymerization, and/or by encapsulation. After promising animal studies, several of these modified Hb solutions are now being studied in Phase III clinical trials. Among them, diaspirin cross-linked haemoglobin (DCLHb) has been used in cardiac and orthopaedic surgery, and for resuscitation of traffic accident victims. The initial results of multicentre trials are now being analysed.