OBJECTIVES: This in vitro study was designed to evaluate both blood and human gingival fibroblast responses on fiber-reinforced composite (FRC) aimed to be used as oral implant abutment material. MATERIAL AND METHODS: Two different types of substrates were investigated: (a) Plain polymer (BisGMA 50%-TEGDMA 50%) and (b) FRC. The average surface roughness (Ra ) was measured using spinning-disk confocal microscope. The phase composition was identified using X-ray diffraction analyzer. The degree of monomer conversion (DC%) was determined using FTIR spectrometry. The blood response, including the blood-clotting ability and platelet adhesion morphology, was evaluated. Fibroblast cell responses were studied in cell culture environment using routine test conditions. RESULTS: The Ra of the substrates investigated was less than 0.1 mum with no signs of surface crystallization. The DC% was 89.1 +/- 0.5%. The FRC substrates had a shorter clotting time and higher platelets activation state than plain polymer substrates. The FRC substrates showed higher (P < 0.01-0.001) amount of adhered cells than plain polymer substrates at all time points investigated. The strength of attachment was evaluated using serial trypsinization, the number of cells detached from FRC substrates was 59 +/- 5%, whereas those detached from the plane polymer substrates was 70 +/- 5%, indicating a stronger (P < 0.01) cell attachment on the FRC surfaces. Fibroblasts grew more efficiently on FRC than on plain polymer substrates, showing significantly higher (P < 0.01) cell metabolic activities throughout the experiment. CONCLUSIONS: The presence of E-glass fibers enhances blood and fibroblast responses on composite surfaces in vitro.