Attempts have been made to find an RBC substitute that will not require crossmatching, can be stored conveniently for long periods of time, can be excreted or metabolized in a reasonable period of time, is relatively nontoxic, and can provide an adequate delivery of oxygen to body tissues and return carbon dioxide to the lungs. Thus far, no perfect answer has emerged. The current two leading candidates have been hemoglobin solutions (free of RBC stroma) and synthetic substances, of which the most promising to date are fluorocarbon compounds. However, major problems still remain. Free hemoglobin can precipitate in the tubules of the kidney or alter renal function. Another difficulty involves a generalized and a coronary artery vasoconstrictor effect. Also, free Hb can interfere with some biochemical tests. Fluorocarbons usually must be oxygenated for maximum effectiveness, most commonly by having the patient breathe 100% oxygen. Elimination of fluorocarbons from the body is fairly rapid (the half-life is about 24 hours), which sometimes would necessitate continued administration. Thus far, none of these blood substitute preparations has proved entirely successful. However, several new preparations are now in clinical trials.