Folding of the major capsid protein of bacteriophage T4 encoded by gene 23 is aided by Escherichia coli GroEL chaperonin and phage co-chaperonin gp31. In the absence of gene product (gp) 31, aggregates of recombinant gp23 accumulate in the cell similar to inclusion bodies. These aggregates can be solubilized with 6 M urea. However, the protein cannot form regular structures in solution. A system of co-expression of gp31 and gp23 under the control of phage T7 promoter in E. coli cells has been constructed. Folding of entire-length gp23 (534 amino acid residues) in this system results in the correctly folded recombinant gp23, which forms long regular structures (polyheads) in the cell.