After blood is removed from an animal, the biochemical parameters begin to change immediately. To ensure that results of laboratory tests accurately reflect the true physiologic or pathologic state of an individual, care must be taken to minimize such artifactual changes. It is commonly thought that delayed centrifugation of avian blood may affect analyte values; however, there have been few studies documenting changes. Likewise, the occurrence of changes in blood analyte values during preparation of a standard serum sample has not been investigated formally. This study was conducted to document the differences between analyte values in plasma and serum processed using common standardized protocols. Paired plasma and serum samples were collected from chickens. Plasma samples were kept on ice and centrifuged immediately, and serum samples were allowed to clot at room temperature for 90 minutes and then were centrifuged. The paired plasma and serum samples were simultaneously analyzed for standard biochemical analytes. Significant differences between plasma and serum values were noted for 10 out of the 17 analytes determined. Briefly, serum values for albumin, albumin-globulin ratio, and potassium were much lower than corresponding values for plasma, and the serum chloride concentration was slightly lower than the plasma chloride value. The serum globulin mean concentration was almost 3 times the value for plasma, and serum values for creatine phosphokinase, calcium, magnesium, and phosphorus were higher than their plasma counterparts.