A number of fuel-block reactivity-worth measurements were performed in Core No. 1 of the thermionic critical experiment. The assembly is bare and neutronically homogeneous, but the geometry is essentially three-dimensional and the dimensions are small. A synthetic transport perturbation method is introduced for the analysis of the fuel-block worths. The agreement between experiment and analysis based upon this method is good. A useful extension of the method would be a relaxation of the first-order perturbation restriction.