A study was made of the antagonistic effect of Ruminococcus flavefaciens on the cellulolytic activity of Neocallimastix frontalis. An extracellular factor inhibiting the cellulolytic activity of the fungus was detected in the bacterial supernatant. The antagonistic factor, which precipitated with ammonium sulphate at 40% saturation, was temperature-sensitive and was destroyed at temperatures above 60 degrees C. After separation by anion-exchange chromatography, sequential precipitation, dialysis and SDS-PAGE, two protein species of 100 and 24 kDa were identified as being involved in this antagonistic effect. It is known whether the proteins are two subunits of a single protein or represent two different proteins. The inhibitory factor, which is not a bacterial cellulase, did not affect fungal growth, but it inhibited the activity of the fungal cellulases.