Recent research in our laboratories has focused on similar problems in bioactive resin-based composites that utilize a unique calcium phosphate, amorphous calcium phosphate, ACP, as the filler phase rather than the conventional silanized glass fillers (1). This type of composite is capable of providing a sustained release of calcium and phosphate ions in aqueous milieus such as exists in the oral cavity (2). Significantly, this type of composite has the potential for repairing and preserving contiguous mineralized tissues such as enamel and dentin (3). While there have been some promising improvements recently (3-6) with regard to mechanical properties, the less than optimal PS and SD, along with DC are still considered shortcomings of these as well as conventional glass-filled composites.This study was designed to determine the effects of a new high molecular mass oligomeric urethane dimethacrylate co-monomer (PEG-U) on PS, SD and DC. In the past, urethane dimethacrylate (UDMA), a commonly used dental monomer, has been blended with 2hydroxyethyl methacrylate (HEMA) in formulating ACP composites, see Fig. 1 for structures of UDMA, HEMA and PEG-U. Specifically, the aim of this study was to assess the effects of the UDMA resin matrix composition on DC, PS, SD and the biaxial flexure strength (BFS) of ACP composites when PEG-U was used in place of HEMA.Disclaimer: Certain commercial materials and equipment are identified in this article to specify the experimental procedure. In no instance does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, Food and Drug Administration or American Dental Association Foundation or that the material or equipment identified is necessarily the best available for the purpose.