The influence of four non-ionic contrast media on red blood cell morphology was analyzed. The newly synthesized monomeric iopentol and the dimeric iodixanol were compared with the monomeric iopamidol and iohexol in both hypertonic and isotonic solutions at constant hematocrit of 10%. All contrast media solutions induced morphologic changes of the red blood cells and with differences between the different solutions. In isotonic solutions and with increasing concentration of contrast medium, echinocytes were produced in increasing frequency. In isotonic solution and with a high concentration of contrast medium, iopamidol, iopentol and iohexol produced 50% echinocytes and 50% stomatocytes while iodixanol produced 90-100% stomatocytes. In original solution the hypertonic iopamidol, iopentol and iohexol solutions induced desiccocyte deformation, while the nearly isotonic iodixanol still produced stomatocytes. In all the 90% volume ratio solutions (contrast medium/blood) rouleaux formation was inhibited regardless of type of red blood cell.