There is currently urgent interest in identifying the species of origin of the components of different animal by-products. In Europe, this interest is expected to increase with authorization of the re-introduction of these proteins into animal feed formulations. The number of validated methods to differentiate the species of origin for most of these products is limited. An easy, inexpensive, and accurate test was developed to determine the cross-contamination of bovine blood or plasma in porcine whole blood and plasma, both before and after spray drying. Agar gel immunodiffusion (AGID), the studied technique, detected the presence of bovine immunoglobulin G (IgG) in porcine plasma and in whole blood at inclusion levels >0.5% (v/v) in all cases. However, detectability was lower in liquid plasma (0.3%, v/v) and in liquid whole blood (0.5%, v/v). No differences were found when cross-contamination was simulated before or after whole blood centrifugation. The method described is reliable and inexpensive, and the samples are easy to prepare. Both minimal laboratory equipment and expertise are required for detection of bovine IgG in porcine blood products at inclusion levels of >0.5% (v/v).