During the past decade there has been a considerable interest in the use of microemulsions, particularly of L2 type (water droplets in oil), for various applications related to biotechnology. To a considerable degree this interest stems from the observation that many proteins can be solubilized in microemulsions based on apolar solvents such as aliphatic hydrocarbons without denaturation or loss of function. This is remarkable in view of the fact that most proteins are sparingly soluble in apolar solvents and that transfer of proteins into these solvents frequently results in irreversible denaturation and loss of biological activity. Wells and his coworkers seem to have been the first to work systematically with proteins in microemulsions (1-3), although the concept of solubilizing a protein in a hydrocarbon solvent had been described earlier (4,5). The majority of papers in this field relate to the use of microemulsions as a medium for enzyme catalyzed reactions but several other uses of microemulsions in biotechnology have also been investigated. This review places an emphasis on enzymatic reactions in microemulsions but it also covers two newer areas of scientific as well as practical interest: use of microemulsion for immobilization of proteins and in bioseparations. In the biologically oriented literature L2 microemulsions are often referred to as reverse micelles. It has been suggested that the borderline between reverse micelles and microemulsion droplets should be defined by the water to surfactant ratio; above molar ratio 15 the system should be referred to as a microemulsion (6). In this review no such distinction is made. All systems containing oil and water together with surfactant are termed microemulsions, regardless of the relative component proportions.