Abstract Fluids from primary human kidney and amnion cell cultures infected with a chick embryo-adapted strain of type 2 poliovirus inhibit the activity of several different viruses. The factor (or factors) responsible for this inhibition is inactivated by treatment with trypsin and is not dialyzable. It cannot be neutralized with type 2 poliovirus antiserum nor sedimented with infectious virus. Attempts to identify it either with ribonucleic acid or ribonuclease have failed. Evidence is presented indicating that the factor interferes with the spread of virus and with viral reproduction. The initiation and maintenance of a chronic infection with type 2 poliovirus in primary human amnion cell cultures depends at least in part upon the presence of this factor. It has been demonstrated in a chronic infection of HeLa cells, but here its role in maintaining the infection has not been finally established.