A colorimetric method for determining protein degradation using a diazonium chromophore was compared with the Dacron bag technique. Diazotized fish meal, maize gluten meal and soya-bean were incubated with strained rumen fluid and a suitable growth medium. Only 49% and 18% of the colour bound to soya-bean was bound to fish meal and corn gluten meal, respectively. Degradation values expressed as a percentage of the total amount actually bound to the dye were in reasonable agreement with dacron bag estimates. However, initial hydrolysis of the diazo feeds was very rapid with little increment in breakdown after two hours incubation. It was concluded that this problem was probably largely a result of limited penetration of the chromophore to potentially reactive sites within the feed particles owing to the large molecular size of the diazonium chromophore.