This protocol describes how to isolate total extracellular RNA from serum or plasma using the Norgen BioTek Plasma/Serum circulating and exosomal RNA purification mini kit. It supercedes an earlier protocol (http://dx.doi.org/10.1038/protex.2015.116). The new protocol is based on our experience vetting the original protocol in multiple labs. The major change is an increase in input volume of biofluid from 200 µL to 500 µL. We found that the larger input volume led to more reproducible amounts of RNA isolation across multiple experiments. We have also included footnotes to explain changes we made to manufacturers' protocols, and observations that were made while carrying out the protocols.