The effect of the ciliates, Eudiplodinium maggii, Diploplastron affine and Entodinium caudatum, and natural protozoal fauna on the ruminal murein concentration and mureinolytic activity was examined on three rams, repeatedly defaunated and refaunated with Eudiplodinium maggii, Diploplastron affine, Entodinium caudatum and natural protozoal fauna. The number of ciliates varied from 18 (E. maggii) to 334 x 10³/g rumen content (natural fauna). The murein concentration fluctuated between 180 and 277 mg/g dry matter (DM). The establishment of ciliates in the rumen of defaunated sheep decreased the murein content by 28-35% (P<0.05). Mureinolytic activity varied from 2.2 and 5.7 μg/g DM of rumen fluid/min and was the lowest in defaunated sheep and the highest in animals faunated with E. caudatum. The protozoa increased this activity from 32 (E. maggii) to 159% (E. caudatum). All examined parameters showed diurnal variations. The ciliate number was the greatest just before feeding and the smallest 4 h thereafter. The fluctuation pattern in murein content was inverse to that of protozoa concentration and mureinolytic activity.