Transgenic animals provide attractive systems for the production of valuable recombinant proteins. Previous studies indicate that milk is a suitable source of secreted recombinant proteins. In the current study, we examine whether excrement can be another source of recombinant proteins by using transgenic mice ubiquitously-expressing green fluorescent protein (GFP) as a model. We found that the surface of excrement from GFP-transgenic mice was fluorescent, and the supernatant after centrifugation of an excrement suspension was rich in undegraded, actively fluorescing GFP. GFP was successfully purified from stool as a fluorescent 27 kDa protein by using a common procedure. Finally, we observed that the fluorescence of digested materials began in the ileum and persisted throughout the remainder of the digestive tract. Our results demonstrate that excrement, which is produced daily regardless of the sex or age of the animal, may be another feasible source of recombinant proteins. The preparation method is simple, economical, and noninvasive. (C) 2018 Elsevier Inc. All rights reserved.