1. Further confirmation of validity of our molecular-biological assay for revertants in OPV (MAPREC) was obtained by increasing database for type 3 vaccine with 65 vaccine lots from 3 other countries which demonstrated complete correlation with the results of tests in monkeys. 2. To identify a mutation(s) in the viral genome responsible for neurovirulence in monkeys, sequencing of the entire genome of seven clones of Sabin type 3 virus has been completed. A clone containing wild-type reversion at position 2493, possesses higher replication ability in vitro, but is not neurovirulent in monkey test. 3. To study mutations which may occur in poliovaccine type 3 Sabin seed virus, sequencing of the entire genome at passage level six in AGMK cells has been performed and several new mutations were found, which accumulate consistently upon passaging. 4. New extremely sensitive modification of MAPREC assay was developed, which enables to detect mutations at a level of a few hundredth of one percent and will be useful for quality control of stability of vaccine lots. 5. International Workshop was organized in December '92, attended by over 150 researchers from 11 countries, with participation of all the leading experts in the field. 6. Preparational work has been done for the WHO International Collaborative Study based on MAPREC method.