A process for screening an expression library to identify clones expressing enzymes having a desired activity is provided. The process involves first generating from genomic DNA samples of one or more microorganisms an expression library comprising a plurality of recombinant cell clones, and then introducing into capillaries in a capillary array a substrate and at least a subset of the clones, either individually or as a mixture. Interaction of the substrate and a clone expressing an enzyme having the desired activity produces an optically detectable signal, which can then be spatially detected to identify capillaries containing clones producing such a signal. The signal-producing clones can then be recovered from the identified capillaries.