Propionibacterium spp. are common flora of human skin. Nevertheless, currently recommended culture procedures do not include anaerobic processing with the result that this organism may go undetected on a colonized catheter. To determine the rate of catheter colonization by Propionibacterium spp., a sample of 1000 vascular catheters was processed by the roll-plate technique and, after conventional aerobic processing, all primary culture plates were reincubated in an anaerobic atmosphere. Propionibacterium acnes was detected in significant counts in the vascular catheters of 39 patients. This represents 14.7% (95% CI, 12.5-16.9) of all positive catheters. Propionibacterium is the second most frequent genus-colonizing catheter tips after Staphylococcus spp. Methodological shortcomings impair the detection and proper adscription of P. acnes as a potential cause of catheter-related infections.