Large bore catheters (n = 42, 31 patients) which remained in position for a period of between 2 to 73 days (mean = 26.8 days/patients) were investigated with raster electron-microscopy (scanning electron micrographic: SEM) and for bacterial colonisation. Microbiological findings indicated colonisation of the intravascular large bore catheters in 8 out of 42 catheters (19%). SEM morphological investigation showed bacterial colonisation in only 3 catheters (7%). The 3 different catheter materials had small deposits of fibrin and protein on the inner and outer surfaces after 2 days. This second layer covered the entire surface after 3 days and increased to a thickness of 3-60 microns in the following days. In this matrix erythrocytes, thrombocytes and granulocytes could also been seen.