We show that it is possible to distinguish between homogeneous and heterogeneous dislocation nucleation on the basis of differences in experimentally measured theoretical strengths. From nanoindentation tests, the critical shear stress for dislocation nucleation in two different Mo-alloy single crystals (Mo-3Nb and Mo-10Al-4Ni) is found to be ~1/8 of the shear modulus. The corresponding stress in uniaxially compressed Mo-10Al-4Ni micropillars is ~1/26 of the shear modulus. This strength difference is due to the higher critical stress required to nucleate a full dislocation loop homogeneously in the bulk as opposed to a half or quarter loop heterogeneously at a surface or edge. PRB 77, 060103(R), 2008.