Polyethylene glycols (PEGs) can either preferentially bind to biomolecules or exert excluded volume effect depending upon their chain length and concentration. We have studied the effect of ethylene glycol (EG) and PEGs of different chain lengths (Mn 400 and 4000) on the enzyme efficiency of hen-egg-white lysozyme (HEWL) on Micrococcus lysodeikticus (M. Lys.) cell. The activity shows a bell-like profile as the turnover number increases from ~1.3 × 105 s−1 M−1 in water to ~1.7 × 105 s−1 M−1 in presence of 2% PEG-400 beyond which it decreases to ~0.7 × 105 s−1 M−1 at 20% PEG-400. Solvent polarity, excluded volume effect, soft nonspecific interactions and structural flexibility are found to be the competing factors which govern the overall enzyme activity as evidenced from circular dichroism (CD) and fluorescence measurements. Thermal unfolding temperature (Tm) of HEWL also shows a bell …