The gene II region of bacteriophage f1 DNA codes for two proteins, the 46 kd gene II protein and the 13 kd gene X protein, which results from an in-phase start at codon 300 of gene II. Using antigene II protein IgG, we show that the intracellular concentration of both proteins is controlled by the phage gene V protein. In wild-type f1-infected cells, the amount of gene II protein reaches a plateau of about 1500 molecules per cell at 20 min after infection, as measured by blot immunoassay. Similarly, the amount of gene X protein reaches a peak of about 500 molecules per cell around 10 min after infection. In contrast, when the gene V protein is inactive, both gene II and gene X proteins continue to accumulate at a high rate for at least 40 min after infection. This difference is caused by decreased synthesis of gene II and gene X proteins in the presence of gene V protein, which represses the translation of these two proteins.