Background: Although much attention is currently directed to studying microbial biofilms on a variety of surfaces, few studies are designed to study bacterial growth on surgical suture. The purpose of this study was to compare the kinetic development of Staphylococcus aureus and Enterococcus faecalis on five surgical suture materials and to clarify factors that might influence this growth. Methods: Pure cultures of S. aureus and E. faecalis were incubated with five types of suture for four days using either tissue culture medium or a bacterial growth medium. Suture-associated bacteria were quantified daily. In selected experiments, the bacterial growth medium was supplemented with heparin, a substance known to promote S. aureus biofilm formation. The ultrastructure of S. aureus biofilm developing on braided suture was studied with scanning electron microscopy. Results: Staphylococcus aureus and E. faecalis were recovered in greater numbers (typically p<0.01) from braided than from monofilament suture, and the numbers of bacteria were greater (often p<0.01) on sutures incubated in bacterial growth medium rather than tissue culture medium. Addition of heparin 1,000U/mL to silk or braided polyglactin 910 suture incubated three days with S. aureus resulted in greater numbers of bacteria on day one but not on subsequent days. Scanning electron microscopy showed a maturing S. aureus biofilm that developed from small clusters of cells among amorphous material and fibrillar elements to larger clusters of cells that appeared covered by more consolidated extracellular material. Conclusions: Bacterial growth was favored on braided vs. monofilament suture, and heparin enhanced bacterial adherence after day one, but not at subsequent times. Staphylococcus aureus adhered to suture material and formed a structure consistent with a bacterial biofilm.