Purified concentrates of the Lansing strain of poliomyelitis virus have been prepared from cotton rat CNS with specific infectivities increased 20,000 times, on the average, above that of the original infected tissue. The fraction of total virus infectivity recovered in the purified concentrates averaged 40 to 50%. The virus in the concentrates of high specific infectivity has been identified by analytical electron microscopy to be a spherical particle 28 mμ in diameter. This identification was made through correlative experiments relating particle counts to infectivity and is discussed in detail. The concentration of the virus in CNS tissue of paralytic cotton rats is calculated to be about 0.2 μg per gram of tissue. It is shown that about 21,000 virus particles or 3.0 × 10-13 g of virus are consistently present in one LD50 inoculum for cotton rats, but it is not known whether all or a constant fraction of these particles are infectious. The electron microscopic evidence shows that the virus is highly uniform in size and shape.