The cohesive plateau theory states that the strength at a bonded interface should equal the strength, cohesively, of the substrate with which it is bonded. A new method was developed to test cohesive strength of dentin so as to offer an opportunity to compare the data against shear bond strength values of commercially available dentinal bonding agents using the same test assembly. The method applied a shear force to a standard circular column of dentin continuous with the tooth from which it was cut with a tubular diamond drill. The mean cohesive strength of dentin in the shear mode, based on 30 specimens, was 36.18 ± 6.81 MPa.