An interlaboratory study (ILS) was performed to evaluate the reproducibility of a new test method to characterize intrinsic threshold stress intensity, ΔK_th,i, as a function of near-tip residual stress, σ*. The new material property carries potential value for improving the reliability of engineering estimates of residual life under spectrum loading for fatigue critical components and structural elements. The method has been shown to considerably reduce the machine time required for estimating ΔK_th,i. The test method involves determining conditions for the onset of fatigue crack growth under controlled near-tip residual stress, σ*. Imposition of controlled σ* at threshold demands the precision application of periodic overload-underload cycles. A variety of measures were put in place to cope with the scope and scale of the ILS, including steps to ensure the material uniformity and similarity of test specimens, the reproducibility of the test method, postprocessing and consolidation of results from individual participants, and finally, the validation and statistical processing of the results of the ILS. Dedicated software was used at all stages of the ILS to ensure reproducibility of the test method across participating laboratories.