The efficiency of selection for catalysis by phage display library was studied using a ligase (the Escherichia coli biotin ligase BirA) and an endopeptidase (the rat trypsin His57->Ala mutant) as model enzymes. These enzymes could be displayed on phage as fusion proteins with calmodulin and the minor coat protein pIII. The display of functional enzyme and the efficiency of selection for catalysis were considerably improved by using phage vectors. In model selection expts., phage displaying BirA were consistently enriched (between 4- and 800-fold) per round of panning, relative to neg. controls. Phage displaying the trypsin His57->Ala mutant were also enriched (between 15- and 2000-fold), relative to neg. controls. A combinatorial phage display library of trypsin mutants was constructed to improve the catalytic properties of the trypsin His57->Ala mutant. Mutants with catalytic properties superior to those of trypsin His57->Ala mutant could not be isolated. [on SciFinder (R)]