Disclosed and claimed are methods for selecting a recombinant virus, phage or cell expressing a catalytic antibody or catalytic portion thereof, or for selecting catalytic activity by a moiety. The method employs reaction-based selection for catalytic activity. The method can also be used to concentrate (increase the proportion of catalytic to non-catalytic moieties) a sample containing a catalytic moiety or viruses, phages or cells expressing a catalytic moiety. The selection or concentrating can be by employing a mechanism-based inhibitor, catalysis-accelerated movement, surface binding, changes in enthalpic component of binding as a function of temperature, or changes in binding by competition, or combinations thereof. The invention also comprehends a method for producing a recombinant virus or a cell-line expressing a catalytic moiety such as a catalytic antibody or catalytic portion thereof; and, this method can include infecting a suitable host with viruses which are screened for the expression. In addition, recombinant viruses and cell-lines so expressing a catalytic moiety such as a catalytic antibody or catalytic portion thereof are also disclosed and claimed.