The lack of a chemical bond between conventional denture base materials and framework elements represents a significant problem in removable prosthodontics. Poor chemical bonding of a denture base resin to cast metal frameworks often introduces adhesive failure and increases microleakage. Purpose. The purpose of this study was to examine the shear bond strength of a denture base acrylic resin to commercially pure titanium alloy (CP Ti) and a cobalt-chromium alloy (Co-Cr) using a hybrid bonding system. Material And Methods. Square plates (of different designs) were cast from the 2 alloys. The plates were grit-blasted with 50 mu m of alumina and treated with the Rocatec (TM) bonding system. A denture base heat-cured acrylic resin was then applied to the plates. Specimens without bonding were also prepared as controls. Both alloys were configured as frameworks with different retentive designs: flat plate, lattice retention, mesh retention and bead retention. Shear bond strength values were determined at a crosshead speed of 0.5 mm/min. Results. For CP Ti plates, shear bond strength was the highest for acrylic resin adhered with Rocatec (TM) to flat plates, followed by the bead, lattice and mesh designs. The shear bond strength for different retentive Co-Cr frameworks, with or without Rocatec (TM), was the highest for bead retention, followed by mesh, flat plate and lattice. There was a statistically significant difference (P <.05) in bond strength between the 2 alloys for both flat plate and lattice retentive frameworks bonded with Rocatec (TM) to acrylic resin. Conclusion. The application of the Rocatec (TM) bonding system significantly improved the shear bond strength of denture base resin using both cast (CP Ti) and Co-Cr alloys. [Fahad A. Al-Harbi and Mohamed Saber A Bond Strength of Poly (methyl methacrylate) Denture Base to castTitanium and cobalt-chromium Frameworks of Different Designs. Life Science Journal 2012; 9(1):610-616]. (ISSN: 1097-8135). http://www.lifesciencesite.com. 90