The purpose of the present microbiologic and case-controlled clinical study was to examine the colonization of 2 different resorbable barrier membranes by sulfate-reducing bacteria (SRB). The barrier membranes tested were Guidor matrix barrier and Resolut regenerative material. Ten patients exhibiting 3 Class II furcation defects and 7 intrabony defects were included in the study. The probing depth and the clinical attachment level at 4 surfaces per tooth were taken at the beginning of the study. Microbiologic samples were taken from the experimental sites and from the approximal sites of the adjacent teeth. Both types of resorbable membranes were positive for SRB colonization. The detection of SRB in 2 of 7 intrabony defects and in all defects with furcation involvement before the membrane placement indicated that these organisms are a common inhabitant of sites showing periodontal destruction and are associated with guided tissue regeneration (GTR). According to the clinical criteria for healing tendencies used in this study, the GTR procedures were less successful in the presence of SRB. There were no significant clinical effects of different resorbable membrane materials or membrane layout on attachment level changes for either the intrabony defect or furcation groups after 18 months. There were no statistical differences for sites that became exposed to SRB when compared to sites that remained unexposed after 18 months. The numeric significance of SRB in relation to the total microbial count needs to be determined to gain insight into the ecologic role of membrane resorption rates.