Different amounts of diatrizoate, ioxaglate, iohexol, iodixanol, NaCl 1,000 mOsm/kg, mannitol 1,098 mOsm/kg, and meglumine (meglumine concentrations corresponding to the content in the diatrizoate solutions) were added to either whole blood or a suspension of granulocytes in autologous plasma, and the adherence to nylon fibers was determined. At high concentrations all the investigated contrast media (CM) inhibited granulocyte adherence. The degree of inhibition was significantly greater when the ionic CM diatrizoate and ioxaglate were used, as compared with the nonionic media. Meglumine solutions at high concentrations also inhibited adherence but significantly less than diatrizoate solutions containing the same amount of meglumine. Diatrizoate showed the greatest inhibitory effect on granulocyte adherence, and significant inhibition could be detected even with a 1.25% solution.