The objective of this study was to investigate the effect of shear flow on the percolation concentration (c(p)) for solutions of fibrillar protein assemblies. Theoretical calculations were performed to obtain c(p) versus Peclet number. They were based on a random contact model for rodlike particles, making use of a shear dependent excluded volume per fibril. We found c(p) to increase with increasing Peclet number. Results of flow birefringence measurements were used to obtain the rotational diffusion coefficient at c(p), which enables one to transform the theoretically obtained c(p)(Pe) into a prediction for c(p) versus shear rate. This prediction was used to fit viscosity measurements as a function of shear rate, near the percolation threshold. A satisfactory fit was found indicating that the percolation threshold, c(p), as function of shear rate can be predicted by combining theory and optical measurements. (c) 2005 The Society of Rheology.