Human endothelial cells in cultures derived from the veins of fresh umbilical cords were incubated for 24 hours with angiographic contrast media added to the cell culture medium. The cells were preincubated with Na2e'Cr)04 for subsequent 5'Cr-release test. Four different concentrations of the contrast media were used. Cell toxicity as judged by measuring the release of 51Cr was, in decreasing order, as follows: Sodium/meglumine diatrizoate, meglumine metrizoate, ioxaglate, iopamidol, metrizamide, iohexol. The highest concentration of sodium/meglumine diatrizoate (Urografin) led to 99 per cent cell death after 24 hours, while the same concentration of iohexol (Omnipaque) induced about 40 per cent cell lysis. The assay system with human endothelial cells thus seems to be useful for evaluation of the toxicity of contrast media. The osmolality was measured in all the solutions. Hyperosmolality accounted for a major part of the cell toxicity, but additional toxic effects of the substances were observed. The strongest osmolality-independent toxic effect was caused by ioxaglate.