The Salmonella typhimurium bacteriophage P22 assembles an icosahedral capsid precursor called a procapsid. The oligomeric portal protein ring, located at one vertex, comprises the conduit for DNA entry and exit. In conjunction with the DNA packaging enzymes, the portal ring is an integral component of a nanoscale machine that pumps DNA into the phage head. Although the portal vertex is assembled with high fidelity, the mechanism by which a single portal complex is incorporated during procapsid assembly remains unknown. The assembly of bacteriophage P22 portal rings has been characterized in vitro using a recombinant, Histagged protein. Although the portal protein remained primarily unassembled within the cell, once purified, the highly soluble monomer assembled into rings at room temperature at high concentrations with a half time of approximately 1 h. Circular dichroic analysis of the monomers and rings indicated that the protein gained α-helicity upon polymerization. Thermal denaturation studies suggested that the rings contained an ordered domain that was not present in the unassembled monomer. A combination of 4,4′-dianilino-1,1′-binapthyl-5,5′-disulfonic acid (bis-ANS) binding fluorescence studies and limited proteolysis revealed that the N-terminal portion of the unassembled subunit is metastable and is susceptible to structural perturbation by bis-ANS. In conjunction with previously obtained data on the behavior of the P22 portal protein, we propose an assembly model for P22 portal rings that involves a meta-stable monomeric subunit.