Estimates of the degradability of protein in the rumen are essential for the application of new systems which have been suggested for the evaluation of the protein requirements of ruminants (Agricultural Research Council, 1980). The effective percentage degradability (P) of protein supplements in the rumen is dependent not only on the course of degradation of the protein particles in the rumen, but also on the time distribution of their stay in the rumen, and will decrease if there is an increase in the rate of passage of the particles. It was shown by Ørskov & McDonald (1979) that if the percentage protein disappearance (p) from samples incubated for time t is described by the equation and if k is the fractional rate of passage from the rumen, then the effective degradability can be calculated as . The calculation depends on the equation for p remaining valid from t =0 (time of ingestion) until a time when all the particles have passed beyond the rumen. It has been found that for some protein supplements, linseed meal for example, the equation for p does not hold true for low values of t. The present note proposes a modification to the formula for effective degradability so that it may remain valid under these circumstances.