A revision to the US Pharmacopeia and the National Formulary (USP-NF) now allows the use of in vitro pyrogen tests in place of the in vivo rabbit pyrogen test (RPT). Effective 1 May, 2017, this change appears in general chapter <151>, “Pyrogen Test,” USP 40 – NF 35. In advance of this effective date, the revised text of USP-NF general chapter <151> has been published to aid companies preparing for compliance. This revision cites the US Food and Drug Administration (FDA) Guidance for Industry: Pyrogen and Endotoxins Testing Questions and Answers which provides examples of in vitro pyrogen assays, including monocyte activation tests and the recombinant horseshoe crab factor C assay.