Serum samples stored in a refrigerator and in ambient room air were studied for alteration of mean weight and mean concentration values. The storage period selected was four hours and the samples were variously stored exposed, refrigerated, and covered with a sample tray cover and by an airtight cover. Some storage procedures produced values more than 2 SD from the mean for many analytes. These findings were confirmed by a parallel study revealing a decrease of sample weight after the four-hour storage period. Storage at 4 C in sample cups covered by an airtight opaque shield alleviated the problem. Failure to treat external quality control sample pools in the same way as other samples will produce unexplained out-of-control results.