This booklet reports a visit in 1959 to the U. S. S. R., Poland, and Czechoslovakia. The visit was made under the auspices of WHO in order to evaluate various aspects of the programs of mass vaccination using Sabin's strains of live poliovirus. In the U. S. S. R., about 15,000,000 persons were so vaccinated. All the evidence pointed to the safety of the procedure for the vaccinated persons and the community throughout follow-up periods of six to nine months. A final evaluation of the effectiveness of the oral vaccine could not be made, but the results were regarded as promising. Although no conclusions were drawn from the observation of the programs in Poland and Czechoslovakia, the report stressed the advantage from the standpoint of the epidemiologist of a large controlled population that is used to submitting to orders from government authorities. This booklet must be considered a preliminary report.