We have studied the maturation of T4 polyheads, the aberrant tubular structures related to the capsid of T4 bacteriophage. Conditions have been found under which more than 95% of the major head protein (P23) undergoes the same cleavage that occurs during development of the normal capsid. The concomitant structural changes in the polyheads have been followed using electron microscope image filtering techniques. As a result of the cleavage, a radical transformation of the hexagonal lattice occurs, involving a 10-15% expansion in the lattice dimensions. However, a metastable intermediate state similar to the uncleaved structure has been observed immediately after cleavage of the protein subunits. Some kind of additional physical stimulus seems to be required to trigger the major structural change, which appears to be highly cooperative.