A rapid screen was developed to test the stability of proteins in ionic liquid–aqueous mixtures using green fluorescent protein (GFP) as a reporter. In at least one ionic liquid (IL), GFP retained 50% or more of its fluorescence in IL volume fractions as high as 75%. ILs that best preserved GFP fluorescence also showed the best retention of cellulase activity. Using this screen, two potential candidates for in situ enzymatic hydrolysis of biomass, 1,3-dimethylimidazolium dimethylphosphate (Mmim DMP) and 1-ethyl-3-methylimidazolium (Emim) lactate, were identified. A commercial Trichoderma reeseicellulase mixture retained activity in both ILs up to 40% (w/w) IL, and β-glucosidase remained active after incubation in 60% (w/w) Mmim DMP for 8 h, indicating the possibility of in situ cellulose hydrolysis in IL–water mixtures.