The effects of antibiotics and Chloroform on the fermentation rates of protein in the rumen and efficiency of rumen microbial growth yield were studied. One hundred ml of rumen inoculum with 100㎖ culture medium in a 250-㎖ serum bottle was incubated for 0, 2, 4 and 8hr. Forty ppm of Avoparcin, Thiopeptin and chloroform were added to each bottle at zero time. Twenty-㎖ samples taken from each incubation bottles were transferred to 50-㎖ centrifuge tubes for the preparation of washed cell suspension. Microbial-N, cell density and NH₃-N production were determined Degradation rates of trypticase and the Y_ATP with or without antibiotics were also calculated. All antibiotics added to rumen inoculum in this study inhibited microbial-N yield as compared with control. Microbial growth rate and the Y_ATP were also decreased considerably with addition of antibiotics. Degradation rates of trypticase were also inhibited from 5.9%/hr control to 3.1-4.0%/hr with the addition of antibiotics. Thus. antibiotics, the rumen additives to rumen inoculum in this study may have interfered with ruminal digestion of protein and growth of rumen microbes.