Regulatory bodies recommend the use of an assay based on the assessment of the endogenous thrombin potential (ETP) for the investigation of the activated protein C resistance (APCr) in the development of steroid contraceptives in women. However, the assays described in the literature are home-made and not standardized regarding the method, the reagents, the reference plasma and the quality controls. In the absence of any commercially available method, we aimed at validating the ETP-based APCr assay. The validation was performed according to regulatory standards. The method targets a 90% inhibition of the ETP in healthy donors in the presence of APC compared to the same condition in the absence of APC. As a large-scale production of a pool of plasma from well-selected healthy donors is impossible, algorithms were applied to a commercial reference plasma to correlate with the selected pool. Repeatability and intermediate precision passed the acceptance criteria. The assay demonstrated a curvilinear dose response to protein S and APC concentrations (R 2 > 0.99). Analysis of plasma samples from 47 healthy individuals (22 women not taking combined hormonal contraceptives [CHC], and 25 men not Factor V Leiden carriers) confirmed the validity of the test, with a mean inhibition percentage of 90%. Investigations in 15 women taking different contraceptives and in two subjects with Factor V Leiden confirmed the good sensitivity and performance of the assay. This validation provides the pharmaceutical industry, the regulatory bodies and physicians with a reproducible, sensitive and validated gold-standard ETP-based APCr assay.