The National Institute of Standards and Technology (NIST), in conjunction with the Centers for Disease Control and Prevention (CDC) and NIH's Office of Dietary Supplements (ODS), initiated an interlaboratory analytical comparison study of total fatty acid concentrations in human serum in May 2012. The first sample was candidate Standard Reference Material (SRM) 2378. SRM 2378 consists of three levels of fatty acids in serum collected from: (1) donors who have not taken fish or flaxseed oil supplements for one month prior to collection; (2) donors who have taken flaxseed oil supplements for a minimum of one month prior to collection; and (3) donors who have taken fish oil supplements for a minimum of one month prior to collection. In addition, participants were requested to run SRM 1950 Metabolites in Human Plasma as a control sample. This is a performance-based program so participating laboratories were requested to use the analytical procedures that they typically use in their laboratories for these analyses and report data for those fatty acids that they typically quantify. The results from the first exercise are reported along with a summary of the analytical methods used.