The goal of the proposed research is to determine the three dimensional structure of poliomyelitis virus at high resolution by X-ray crystallographic methods. Poliovirus is a member of the enterovirus group of the picornavirus family. The picornaviruses, and poliovirus in particular, have been well characterized biologically. Poliovirus thus serves as a excellent model system for understanding the structural basis for several important properties of animal viruses including: assembly, antigenic properties, mechanism of neutralization by antibodies, recognition by host cell receptors, and the mechanism of penetration and entry into susceptible cells. Crystals have been obtained from both the Sabin (attentuated) and the Mahoney (virulent) strains of the type I serotype of poliovirus. To date, the crystals of the Mahoney strain have proved more suitable for X-ray crystallographic studies. The crystals of the Mahoney strain belong to the space group P21212 with unit cell constants a=324 A, b=359 A, c=381 A and diffract to at least 2.2 A resolution. The structure determination will use methods devised and tested for previous successful crystallographic studies of plant viruses.