A sensitive and simple immunoassay was developed to screen Escherichia coli transformed with recombinant DNA plasmids carrying a cellulase gene. The assay was used to identify a recombinant DNA plasmid carrying at least one cellulase gene from Cellulomonas fimi. The enzyme present in extracts of E. coli carrying the plasmid was active in catalysing the hydrolysis of carboxymethylcellulose as indicated by the production of reducing sugars.