Solutions of the nonionic monomeric contrast medium iohexol (300 mg I/ml) with and without added NaCl were investigated for effects on red blood cell aggregation and blood coagulation. Three volumes of a test solution were mixed in test tubes with one volume of human blood. During 30 min samples of the mixture were taken for investigation. Six test solutions were used: 1) iohexol, 2) iohexol+glucose 280 mM, 3) iohexol+NaCl 150 mM, 4) glucose 280 mM, 5) glucose 140 mM+NaCl 75 mM, 6) NaCl 150 mM. Test solutions with NaCl caused no aggregation. Test solutions without NaCl always caused macroscopic red cell aggregates. These aggregates always disappeared when saline was added to the sample. The macroscopic red cell aggregates could be dispersed to microscopic aggregates by shaking the test tubes. During the next 30 min macroscopic aggregates returned in the glucose solution but not in the iohexol solutions. In 30 min, blood mixed with iohexol solutions never coagulated while blood layered on top of the same iohexol solutions always coagulated. Blood mixed with solutions 5 and 6, both without iohexol, always coagulated. It is concluded that adding 150 mM NaCl to iohexol did not eliminate its ability to anticoagulate whole blood, but inhibited its ability to aggregate red cells. This inhibition was not caused by the osmotic effects of the added NaCl.