The influence of surface free energy on supra- and subgingival plaque microbiology was examined in 9 patients with functional fixed prostheses supported by endosseous titanium implants. Two abutments (trans-mucosal part of the 2 stage implant) were replaced by either a new titanium abutment or a fluor-ethylene-propylene (FEP) coated abutment per subject. After 3 months of habitual oral hygiene, plaque samples were taken. Supragingival plaque was examined by means of differential phase-contrast microscopy, whereas for the subgingivial plaque additional analyses (DNA probes analysis, culturing) were performed. The subgingival samples were taken by paper-points and by scraping of the subgingival abutment surface. Differential phase-contrast microscopy showed a significant difference in plaque composition, especially when supragingival plaque was considered (P = 0.05). FEP coated abutments frequently harbored more coccoid microorganisms, whereas spirochetes or motile organisms were only detected around titanium abutments. Subgingivally, the number of colony forming units (CFU) in paper-points was comparable for both types of abutments. If the to-the-abutment-adhering plaque was considered, the number of CFU was 5 times higher on the titanium abutments than on the FEP coated abutments. However, this difference did not reach a statistical level of significance (P = 0.38). The DNA probe analysis of the subgingival plaque collected with paper-points showed a slightly higher frequency and concentration of perio-pathogens around the titanium abutments. However, the inter-substratum differences were smaller than the inter-subject differences. The latter seemed to be related to patient's dental status.(ABSTRACT TRUNCATED AT 250 WORDS)