AbstractAntiviral activities of known protease inhibitors were assayed in virus‐infected cell cultures. Some members of the cystatin superfamily, in particular chicken cystatin, were able to block virus replication. In a binding assay, using purified components, chicken and human cystatin were able to bind poliovirus protease with affinities which were reflected in their relative anitviral potencies. Prospects for application of protease inhibitors in clinical viral infections are discussed.