The mechanism of adherence of Staphylococcus epidermidis to commercially available catheters was studied in vitro in a quantitative assay employing 3H-labelled bacteria. It was found that adherence to Teflon catheters was significantly related to the degree of hydrophobicity of the strains. When hydrophobic groups were removed from Staphylococcus epidermidis by pepsin treatment, adhesion was almost completely abolished. Preincubation of catheters in human serum also caused a 80-90% reduction of adherence. Preincubation of Staphylococcus epidermidis in serum similarly decreased adhesion. This effect of serum was mainly due to albumin, while IgG and fibronectin were less effective. Culture of Staphylococcus epidermidis in subinhibitory concentrations (0.5 MIC) of cephalothin, clindamycin and vancomycin resulted in a 30-80% reduction in adhesion.