A comparative evaluation of the determination of acid-detergent fiber (ADF) according to Van Soest and of a modified procedure for the determination of detergent fiber (DF) was performed on 54 forage samples. Losses of cellulose which occur during ADF preparation can be avoided by the DF procedure. True cellulose contents were calculted from ADF and ADL [acid detergent lignin] because of nonlignocellulosic contaminants compensating numerically for the cellulose losses.