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It has been shown in the last few years that enzymatic reactions can be carried out efficiently in nearly equimolar mixtures of substrates in the absence of added solvents (1,2). However, for solvent-free reactions to be efficient, a reasonable degree of miscibility between substrates is usually required. This limits the practical utility of this approach to biotransformations of substances that are either liquid at the reaction temperature (e.g., lipids [see other chapters in this volume]) or can be chemically "engineered" to become soluble in the other liquid substrate (3-9). In order to overcome this problem, we have developed a complementary bioreaction system where enzymatic transformations take place in the liquid phase of a low-melting-point eutectic mixture of substrates (10-14). Such eutectics can be formed by mixing molten substrates together at elevated temperatures and then cooling the mixture down to an ambient temperature (11). A small amount of organic solvent or water is often used to further suppress the melting point and/or to decrease the viscosity of the reaction medium (12). Enzymatic transformations in eutectic mixtures have been successfully used for the preparation of numerous peptide derivatives (10,13,14). [ABSTRACT FROM AUTHOR]