A method for identifying and sorting variants of a chosen enzyme is disclosed. Enzyme variants of a chosen enzyme are obtained and then linked to their corresponding genetic code through any of a family of suitable surface display methods. The enzyme variants, now displayed on the surface of a biological particle such as a phage, virus, yeast, or bacterium are then encapsulated in a vesicle containing an enzyme activity-sensitive assay reagent. The enzyme variant is thus exposed to the assay reagent, and displays a signal using the enzyme activity-sensitive assay reagent in a manner proportionate to the levels of activity of the enzyme, thus rendering the vesicles suitable for mechanical sorting based on these levels. The vesicles are then sorted using methods known in the art to isolate those variants exhibiting possibly beneficial variations in enzyme function.
 This invention was made with Government support under Contract Number W-7405-ENG-36 awarded by the United States Department of Energy to The Regents of the University of California. The Government has certain rights in the invention.