An in vitro study was conducted to examine the degradation of various protein sources by mixed rumen bacteria, mixed rumen protozoa and a combination of bacteria and protozoa. Rumen microorganisms were isolated from the rumen of sheep fed a concentrate mixture and hay. Microbial suspensions were anaerobically incubated at 39°C for 12 h. The results revealed that, mixed rumen bacteria always produced more ammonia than mixed rumen protozoa. Heated soybean meal was deaminated at a faster rate (P<0.05) than that of soluble casein, insolube casein and killed bacteria by bacteria. However, protozoa deaminated killed bacteria at a faster rate (P<0.05) than that of soluble casein, insolube casein and heated soybean meal. Nonammonia-nonprotein N was greater (P<0.05) with protozoa than bacteria. When incubations containing bacteria or protozoa were compared with the combination of bacteria and protozoa, the combination always caused a synergistic increase in ammonia and decrease in nonammonia-nonprotein N (P<0.05). It could be concluded that there is synergism between rumen bacteria and protozoa on the degradation of protein.