Animal by-products of category 3 after proper processing may be a valuable fertilizer for agricultural purposes. However, they can contain numerous bacterial and viral pathogens and, in cases of improper handling, may pose a health risk for people and animals. This study is aimed at monitoring the number of fecal streptococci introduced into carriers of different types imitating fragments of meat and bone wastes during composting process in a drum bio-reactor. Fecal streptococci are indicator microorganisms, and are known by their thermoresistant characteristics, so it was assumed that their elimination will also diminish the pathogenic microorganisms present in wastes. Three research cycles were carried out in a drum-type bio-reactor, and a different course of temperature was noted in each of them. In cycle 1, in which the temperature exceeded 60^oC, fecal streptococci died the fastest, 139.0-154.4 hours later (depending on carrier type). In cycles 2 and 3, maximum temperatures were similar (57.2^oC and 58.8^oC, respectively), but secondary multiplication of the streptococci in the 102nd hour of the processes was observed. In cycle 2 at this time their number was similar to the level of initial suspension. The type and size of the carriers were of no major importance to streptococci survival in the bio-reactor. Yet in each of the cycles analyzed, effective reduction was accomplished and the product obtained can be considered to be environmentally safe.