The effect of hydroxyethyl starch (HES), a complex polysaccharide colloid plasma expander, on the structure of fibrin gels was studied turbidimetrically. HES at concentrations as low as 2.5 mg/ml shortened the lag phase of turbidity increase and caused increased fibrin fiber mass/length ratios in both purified fibrin and plasma systems. For purified fibrin gels clotted with thrombin the mass/length ratio increased in a linear fashion from 2.6 to 6.1 X 10(12) daltons/cm as HES increased from 0 to 15 mg/ml. Fiber mass/length ratios for purified fibrin gels clotted with reptilase increased from 1.1 to 6.4 X 10(12) daltons/cm over the same HES concentration range. The addition of 5 mmol/L calcium to either system produced additional increases in fiber size. In 1:10 dilutions of plasma, HES shortened the lag phase, enhanced the rate of turbidity increase, and increased the final gel turbidity. The fiber mass/length ratio for plasma gels clotted with thrombin increased from 0.3 to 1.7 X 10(12) daltons/cm as HES increased from 0 to 20 mg/ml. For plasma gels clotted with reptilase, the fiber mass/length ratio increased from 1.5 to 2.4 X 10(12) daltons/cm over the same HES concentration range. The effects of HES are comparable to those of dextran at concentrations less than 5 mg/ml and can be explained on the basis of HES-enhanced lateral association of fibrin fibers during polymerization.