Large-bore catheters for extracorporeal detoxification methods without and with treated surface with silver or silicone were investigated after removal with a scanning electron microscope and for bacterial colonization. In 42 large-bore catheters of three different materials, small deposits of fibrin and protein on the inner and outer surface were seen. This second layer covered the entire surface after 3 days and increased to a thickness of 3 to 60 microns during the following days. Bacterial colonization was observed in 38.1%. In contrast to these results, the catheters with the treated outer surface showed a very low thrombogenicity and a low contamination rate of 6.7%.