Iotrolan, a nonionic, hexaiodinated dimer, is an extremely hydrophilic compound (P = 0.005). Due to its larger Stokes' radius compared with monomeric compounds such as metrizamide, the diffusion time through membranes is extended. Iotrolan deforms erythrocytes only minimally. There is practically no binding to plasma proteins. The new contrast agent has been shown to exert a very limited effect on the complement system (in vitro); it does not inhibit lysozyme (a standard enzyme) in concentrations less than 100 mg I/ml. To inhibit activity of the enzyme collagenase, much higher concentrations of iotrolan than of metrizamide or iopamidol are needed and this could offer an advantage when used for diskography preceding diskolysis with collagenase. After a single intravenous injection in rats, iotrolan has an LD50 of 28.3 g I/kg - the best general tolerance known for water-soluble contrast media thus far. The superior tolerance of iotrolan compared with iohexol and iopamidol (p less than or equal to 0.05) in rats is statistically significant. On the basis of preclinical experience, iotrolan is a very promising contrast medium for intrathecal and intravascular use.