In four change-over experiments, wilted grass silages, differing in growth stage at harvesting and as a consequence in cell wall content, were fed ad libitum to dairy cows in early and late lactation. Ruminal degradation rate of the crude protein fraction of the silages was investigated using nylon bag incubations. No significant relation between the degradation rate (%h-1) and the cell wall content of the silages was found. The soluble and undegradable fractions of the crude protein both increased with an increase in silage cell wall content. The soluble fraction was more closely related to dry matter content and date of harvesting than to cell wall content of the silages. The fraction of dietary protein escaping rumen fermentation increased with cell wall content.In duodenal protein, the fraction originating from the diet, estimated using amino acid profiles of dietary, microbial and duodenal protein, decreased with cell wall content.