Interlaboratory comparison studies were directed toward the measurement of 24 fatty acids in selected serum and plasma matrices. Frozen or freeze-dried serum or plasma samples together with control materials were sent to laboratories for analysis. Results were then returned to NIST for data evaluation and tabulation. Value-assignment of the samples was based on the median of all the laboratory results, with confirmation based on measurement at NIST using one or more different methods. Consultation and troubleshooting regarding methods of analysis were also provided. A report of exercise results was sent to participants that summarized the results from each comparability study.