The assembly of the product of bacteriophage T4 gene 23 (gp23), the uncleaved form of the main shell protein, has been studied. Assembly and disassembly follow the predictions for entropy-driven processes; assembly is strongly favored by conditions of high salt concentrations and high temperatures, whereas low salt and low temperatures promote disassembly. In the absence of the scaffolding core proteins in vitro, only polyheads, the tubular variant of the prohead, are produced. Kinetic studies show that the rate of polyhead dissociation depends on the concentration of associated protein, not on the number and length of the particles. Comparable to crystal formation, assembly of gp23 occurs above a critical concentration, which is dependent on salt concentration, pH and temperature. These characteristics are common to most self-assembling systems. The oligomeric states of gp23 have been investigated by analytical ultracentrifugation, which indicated the existence, at very low salt concentration and low temperature, of an equilibrium between monomers and higher oligomers, culminating in the hexamer. At pH 9.0 polyheads are completely dissociated into their monomeric gp23 subunits. Our data suggest that the hexamer is a true intermediate of polyhead assembly.