HPV16 infection is reduced in the presence of cysteine protease inhibitors. Infection of 293 cells alone, with HPV 16 DsRed reporter-virions (mock control, Infected), or in the presence of: 20 mM of lysosome neutralizing agent NH4Cl; 10 μM of a non-permeable cysteine protease inhibitor E64; 10 μM of the permeable cysteine protease inhibitor E64-d; 6 μM of the permeable intracellular cathepsin B inhibitor CA-074ME; or 10 μM of the cathepsin L inhibitor. Infection was analyzed and compared 48 hours post binding by FACS of DSRED expression. Inhibitors were present for the duration of infection. Cells alone were analyzed for background fluorescence. Statistics were analyzed by 1 tailed t-test and found to be significant at P < 0.05.