Fiber optic Raman spectroscopy is used for in situ monitoring of supersaturation during the hanging-drop crystallization of aprotinin. Schwartz and Berglund (1999) previously demonstrated this technique for lysozyme crystallization and showed it combines two critical elements for protein crystallization studies: real-time monitoring/control of supersaturation and small amounts of sample. Experiments were carried out using 10 L of protein solution. A partial-least-squares (PLS) calibration based on Raman spectra of standard solutions allowed an accurate measurement of aprotinin in a range of 2-100 mg/mL with a standard error of 0.54 mg/mL determined by a leave-one-out cross validation. A 10× microscope attached to a Raman fiber optic probe allowed the monitoring of the hanging-drop liquid phase in a noninvasive and real-time mode. Aprotinin solubility determined by measuring the protein concentration of drop solution at equilibrium decreased with increase in NaCl concentration. By continuously collecting Raman spectra of the liquid phase in the drop, the protein concentration was monitored in real time during the whole process. Control of supersaturation by manipulating the evaporation rate of the drop solution allowed the optimization of the process, leading to an increase in the resulting crystal size.