This material was formulated from a donation of window period plasma, in order to obtain a suitable concentration the window period donation was then further diluted into negative human plasma. Prior to dilution, the window period donation was seen to contain some very small white lipid particles that could not be removed by centrifugation. In our experience, this is a frequent occurrence in human plasma especially when the material is exposed to freeze thaw cycles which was the case for this donated material. The severity of the particulate presence is also influenced by the diet of the donor, in the case of negative plasma used for diluting purposes, there is a plentiful supply we can be selective and exclude any pack that shows lipid presence, however due to the rarity of the window period donation we could not do this. The final bulk material was mixed thoroughly before and during the dispensing process. The material was then assessed by multiple laboratories in the collaborative study to establish the standard and in accelerated thermal degradation studies in house, in the data sets associated with this work, we have not seen any evidence to suggest that these lipid particles have caused any detriment to the results given in any assay when the material is fully mixed immediately prior to use and particples are inlcuded in the assay sample. The 5th WHO International Standard for hepatitis C virus (HCV), NIBSC code 14/150, is intended to be used for the calibration of HCV secondary standards. The standard comprises genotype 1a HCV antibody-negative, HCV RNA-positive plasma, diluted in pooled human plasma. The virus stock was tested and found negative for HIV-1 RNA, HBV DNA, HAV RNA and parvovirus B19 DNA. The pooled human plasma diluent was sourced from blood donations and had been tested and found negative for HIV antibody, HCV antibody, HBsAg, syphilis antibody, HTLV antibody, as well as HIV and HCV RNA. The standard has been lyophilized in 1.1 mL aliquots and stored at -20 °C. The material has been calibrated in International Units (IU) against the 2nd WHO International Standard for HCV.