Gene product 8 (gp8, 344 amino acids per monomer) of bacteriophage T4 is one of the baseplate structural proteins. We constructed an expression vector of gp8 and developed a method for purification of recombinant protein. CD spectroscopy showed that gp8 is an alpha/beta type structural protein. Its polypeptide chain consists of nearly 40% beta-structure and 15% alpha-helix. These data agree with results of prediction of secondary structure based on the amino acid sequence of the protein. The sedimentation coefficient under standard conditions (S20,w) is 4.6S. Analytical ultracentrifugation results demonstrated that gp8 in solution has two types of oligomers--dimer and tetramer. The tetramer of gp8 may be included in the wedge (1/6 of the baseplate), and the dimer may be an intermediate product of association.