Objective: In the present study two different fiber reinforced composites (glass and polyethylene FRC) (Glass FRC and PE FRC), a restorative particulate filler composite (RC) (Z-250; 3M Dental Products, St. Paul, MN, USA) and a high-leucite ceramic material used for ceramic fillings and crowns (Cer) (Finesse All-Ceramic; Dentsply, York, Pennsylvania, USA) were compared with respect to early plaque formation in vivo.Methods: Disc shaped specimens were randomly distributed among upper first and second molars of fourteen healthy adult volunteers. Plaque samples were collected 24 hours after the attachment of the specimens. Mutans streptococci (MS),non-mutans streptococci (Non-MS) and total facultative bacteriawere cultured.At first stage the data was subjected to Friedman test and subsequent comparisons between materials were done using Wilcoxon signed ranks test. Results: The mean logarithmic CFU counts of studied bacteria recovered from the plaque samples (± SD) are shown in the table.

	Cer	RC	Glass FRC	PE FRC
MS	1.0 ± 1.34	1.4 ± 1.28	1.7 ± 1.25	2.4 ± 1.19
Non-MS	5.7 ± 0.65	5.8 ± 0.68	6.0 ± 0.41	6.5 ± 0.51
Tot facultative	6.0 ± 0.72	5.9 ± 0.71	6.1 ± 0.51	6.4 ± 0.58

The plaque recovered from polyethylene FRC harboured significantly more mutans streptococci than the plaque of ceramic (p = 0.008), restorative composite (p = 0.006) and glass FRC (p = 0.050).Differences between materials were found also for the counts of non-mutans streptococci (p=0.002) and total facultative bacteria (p = 0.012). Polyethylene FRC showed the highest counts and ceramic a trend towards lower counts. Conclusion: It was concluded that in the oral environment polyethylene FRC promotes plaque accumulation and adhesion of mutans streptococci more than glass FRC, restorative composite and dental ceramic. Glass FRC resembles restorative composite with respect to plaque accumulation and adherence of mutans streptococci.