A qualitative assay which can be adapted to screen large numbers of Escherichia coli colonies for the presence of soluble enzymes is described. In a test of the system using a new, especially sensitive assay for isocitrate dehydrogenase activity, colonies producing the enzyme could be correctly identified at the 70% level after 2 h of incubation and at the 100% level after 8 h of incubation. The completed reactions are stable for several days at room temperature.