The purpose of this study was to determine whether the fibrin sheath that develops around an intravenous silicone catheter influences catheter-related sepsis. A rat model with a central intravenous silicone catheter was used. Staphylococcus aureus, dose 1 x 10(7) colony forming units (cfu), were injected via the tail vein, either immediately after catheter insertion (group 1, n = 23) or after the catheter had been in situ for at least 7 days (group 2, n = 22). Blood cultures were done on at 24 hours and 7 days. Animals were killed on day 7 and the catheter was removed for culture (Maki and broth) and scanning electronmicroscopy (SEM). The was no significant difference (P > .05) between the number of positive blood cultures in groups 1 and 2 at 24 hours (16 v 9) and 7 days (12 v 6). In group 1 there were significantly more positive catheter cultures by both methods (23 v 16 in group 2; P < .05) and more cfu/per centimeter catheter (group 1 mean, 520, range, 197 to 600; group 2 mean 195, range 9 to 600; P < .001). In group 1, 12 animals had catheter sepsis compared with 5 in group 2 (P = NS). On SEM a fibrinous sheath was identified on all catheters removed on day 7 but not on 5 catheters inserted and removed after 10 minutes.(ABSTRACT TRUNCATED AT 250 WORDS)