Plasmids harboring the amino-terminal part of bacteriophage f1 gene II confer to bacterial cells partial resistance to infection with the male-specific bacteriophages f1 and f2. This effect (IP-2 phenotype) is due to the production of large amounts of an approximately 20,000-dalton polypeptide corresponding to the amino-terminal part of gene II protein. These results have allowed the isolation of clones producing functional gene II protein in large amounts. An in vitro assay has been developed to test the enzymatic activity of the gene II protein produced.