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The applicability of the plant enzyme ficin (EC 188.8.131.52) for the prediction of protein degradability of ruminal feeds was examined. Optimum pH and the effect of enzyme concentration, incubation time and amylase pretreatment for high-starch feeds on protein solubility with ficin (FS) were determined. The predictive ability of ficin assay in the conditions specified for particular groups of feeds was estimated using 61 feeds including protein meals (7), cereal grains (2), silages (9), sugar beet pulp, dried forages (29) and compound feeds (13) of known in situ effective protein degradability (ED) determined with cows. Optimum incubation time was 2-3 h, enzyme amount (per 100 mg of crude protein in feed sample) equivalent to 9 U for compound feeds and concentrate ingredients not treated with amylase, 18 U for forages, Close linear relationships between the in vitro FS (X, and in situ estimates of ED (Y, were found. The regression equation for all samples (n = 61) was Y = 1.018 X + 1.331, R(2) = 0.92 (P <0.001), SEE 4.17. After splitting to forages (FOR) and compound feeds and ingredients (CON) SEEs were 4.25 and 3.56 respectively. FS values accounted for 85, 92 and 95% of the variation in ED measured in situ for FOR, FOR + CON and CON samples, respectively.