This study investigated the important role of agitation in obtaining consistent and reproducible results when moving from preliminary qualitative screenings for protein crystallisation to quantitative batch crystallisation experiments. Lysozyme-thaumatin binary protein mixture was used as the model protein system in this study. Poor reproducibility between batches were observed for non-agitated crystallisation conditions even if the same sampling timing and frequency applied. With agitation, from 0 to 200 rpm investigated in this study, improved reproducibility of protein crystallisation was observed with increased agitation. Additionally, agitation also had impacts on supersaturation exhaustion rate, yield and crystal size. Moreover, in agitated batch crystallisation, it was found that target protein crystallisation process was decelerated in the presence of protein impurity. In conclusion, we emphasised the essential role of agitation in protein crystallisation experiments else misleading conclusions with inconsistency might be drawn from non-agitated systems.