Heparin surface coated hemofilters and tubing sets were evaluated in comparison to identical but uncoated controls in 8 bovine experiments (74+/-6 kg). No heparin was given (neither systemically nor in the priming fluid). The hemofilters were primed with one liter of Ringer's lactate in both groups and the maximal filter performance (arterial line pressure 300 mmHg; transmembrane pressure (TMP) 500 mmHg) was measured over 6 hours or until filter occlusion. All coated and one control filter remained functional during the scheduled 6 hours. The mean filter patency was 360+/-0 minutes for coated versus 210+/-99 minutes for uncoated (p less than 0.01). Mean blood flow at 1 hour and 6 hours was 675+/-114 and 580+/-96 ml/min for coated versus 432+/-183 and 25+/-43 for uncoated (NS; p less than 0.01). Mean filter output during the 6th hour and total filter output over 6 hours was 4225+/-998 ml and 21779+/-4273 for coated versus 400+/-692 and 7717+/-9757 for uncoated (p less than 0.01; p<0.01). Mean lactatedehydrogenase (LDH) levels before and 30 minutes after hemofiltration were 1855+/-411 IU and 2007+/-635 for coated versus 2160+/-411 and 1945+/-500 for uncoated (NS; NS). The heparin coated hemofilters demonstrated improved thromboresistance resulting in superior filter performance. There was no evidence of increased blood trauma.