Acentric chromosome fragments produced in cells by irradiation or other agents give rise to micronuclei in daughter cells. The micronuclei can be readily counted in large numbers of cells thereby providing a sensitive measure of chromosome aberrations. Previous studies have shown a consistent elevation of micronuclei count following the use of diatrizoate contrast materials. This study was undertaken to compare the micronuclei counts of patients receiving sodium iothalamate with those receiving sodium diatrizoate. Our results indicate that sodium diatrizoate produced significantly greater cytogenetic damage than the sodium iothalamate agents.