In order to perform biological monitoring of exposure to radiation and contrast media, we evaluated the micronucleus count (MN) and the mitotic index (MI) in peripheral blood lymphocytes from patients undergoing excretory urography with diatrizoate (20 patients) and ioxaglate (20 patients). Three samples were taken for each patient: A (before exploration), B (immediately after exploration) and C (7 days later). There were no significant differences in the radiation doses received, nor in the dose of contrast agent, between both groups. The micronucleus count increased significantly in sample B in both groups, the increase being more statistically significant in the diatrizoate group (p less than 0.01) than in the ioxaglate group (p less than 0.05). One week later, the MN were still slightly high (p less than 0.05) in the diatrizoate group only. These results suggest a clastogenic effect which depends, to a great extent, on the nature of the contrast medium.