Summary. Advances in production technologies of factor (F)VIII concentrates during the last two decades has resulted in very pure and safe products. In assessment of recombinant FVIII:C, inconsistent assay values are found comparing one-stage assays with two-stage (e.g. amidolytic) methods. Such discrepancies have been quite prominent in the case of a B-domain deleted recombinant FVIII (BDDrFVIII, ReFacto®). In order to alleviate this assay variance, a product-specific reference standard [the ReFacto Laboratory Standard™ (RLS)], was established for laboratory use with either one-stage clotting or chromogenic substrate assays for the measurement of FVIII:C in ReFacto-containing patient samples. The primary objective of the current study was to assess, under field laboratory conditions, the accuracy and precision of the one-stage clotting assay for the determination of FVIII:C in ReFacto-containing samples employing the new concentrate standard. A secondary goal was to assess whether use of the RLS would minimize the discrepancy between one-stage clotting and chromogenic substrate assays. Thirty-one clinical laboratories worldwide participated in the study of severe-hemophilic plasma (SHP) samples that had been spiked with ReFacto to target levels of 0.9, 0.6 and 0.2 IU mL−1. FVIII:C levels were determined against both the RLS and the local in-house plasma standard (IHS). The results showed good agreement between laboratories in FVIII:C levels obtained by one-stage clotting assays utilizing the RLS, and a good degree of accuracy was found compared with the intended target values. Consistent with previously published data, a discrepancy of approximately 30% was observed between one-stage clotting and chromogenic potencies when the IHS was used as the calibrator. The discrepancy between one-stage and chromogenic assay methodologies was significantly reduced when the RLS was employed as calibrator in the one-stage assay. In conclusion, the study demonstrates that accurate and precise FVIII:C results can be obtained for ReFacto-containing SHP samples by clinical laboratories using a product-specific standard in one-stage clotting assays. In addition, the product-specific reference standard significantly reduced the discrepancy between the one-stage clotting and the chromogenic substrate assay for ReFacto.