The aim of this study was to determine the dynamics of bacterial contamination of feedstuffs during ruminal in situ incubation and to quantify the effects of bacterial contamination on the ruminal in situ degradation of crude protein (CP). Two different approaches using ¹⁵N as a marker were followed, namely: 1. ¹⁵N-labelled rumen bacteria in combination with unlabelled feedstuffs and 2. ¹⁵N-labelling of feeds while leaving rumen bacteria unlabelled. Using the nylon bag technique, a total of 31 feedstuffs was incubated in the rumen of 3 steers. Both marker methods gave in principle similar results. The course of bacterial contamination was best described by an exponential function of the general type A=A_max [1–exp (-C·t)], where A denotes bacterial contamination (% of residue N), A_max is the maximum of bacterial contamination for t ≈ ∞, C is the rate of contamination [%·h^{- 1}] and t denotes the incubation time (h). The A_max values of most cereals ranged from 32 to 38% of the residue N. For roughages and straw, Amax reaches 45 to 100%. A_max, but not C, is significantly positively influenced by the NDF content in feedstuffs (P=0.049). The CP content of feedstuffs negatively affects C (P=0.006). The proportion of bacterial CP of total CP within the in situ bag residues was used to correct the apparent CP degradation rate. This correction was numerically lowest for protein concentrates and grains (0 to 3%) and highest for roughages (up to 50%). In conclusion, correction of ruminal protein degradation for bacterial contamination is necessary, especially for roughages. For cereal grains and protein concentrates it seems to be irrelevant.