This paper examines the bond strength of cement grout plugs cast in rock. The bond strength has been determined through push-out testing in which an axial load is applied to cement grout plugs emplaced in boreholes in welded tuff cylinders. The push-out tests have been performed as a function of borehole (plug) radius and plug length. The use of four different borehole radii and three different plug length-to-radius ratios enables the evaluation of size effects. A power law obeying strength decrease with increasing plug radius and decreasing plug length results. The exponents of the power law extrapolation are about 0.5 for the borehole (plug) radius size effects and about 1.1 for the size effects on plug length-to-radius ratio. The mean axial strength, mean bond strength and mean peak shear strength range from 15.0-170 MPa, 3.90-11.0 MPa and 17.0-115 MPa, respectively. The tensile stresses in and near the vicinity of an axially loaded borehole plug are analyzed. Implications of the tensile stresses and peak shear stresses on plug design are discussed. The main conclusion is a recommendation to design friction plugs in shafts, drifts, tunnels, or boreholes with a length-to-radius ratio of at least eight. (C) 1997 Elsevier Science B.V.