Objective: To compare the adherence of Prevotella melaninogenica and Enterococcus faecalis to 3 guided tissue regeneration membranes: Atrisorb, Lambone, and OsseoQuest. It was hypothesized that OsseoQuest would show increased bacterial adherence compared to Lambone and Atrisorb. Method and Materials: The barriers were suspended in trypticase soy broth containing an inoculum of either P melaninogenica or E faecalis. The samples were incubated under appropriate conditions for 6, 24, and 48 hours. Following incubation, each membrane was mixed in fresh media in a vortex machine to dislodge adherent bacteria. The vortexed media was quantitatively assessed using serial dilutions for viable cell count. Results: E faecalis exhibited higher adherence compared to P melaninogenica with time. Of the membranes tested, Lambone displayed the least bacterial adherence. Conclusion: An analysis of the results indicated that bacterial adherence was time-dependent for all membranes. Membrane structure, chemical configuration, hydrophobicity, and bacterial cell surface structure were suggested as factors contributing to variance in bacterial adherence. Schlagwörter: bacteria, barrier membrane, periapical surgery