Abstract
The properties of naturally occurring poliovirus strains of each serotype have been studied using three marker tests: the R.C.T.40 test, intratypic serodifferentiation and, for type 1 strains only, inhibition by dextran sulphate.The results show that while most strains resemble the prototype strain of the same serotype, considerable natural variation exists, especially in the ability to grow at temperatures between 39° and 40° C.No correlation has been found between the results of the tests on individual strains and it is concluded that no single test is sufficient to characterize a strain.