Association of gp10 and gp11 (gp=gene product) is the first step in the assembly pathway of the wedge part of the baseplate of bacteriophage T4. The gp10-gp11 complex constitutes the six tail pins at the corners of the baseplate hexagon on the distal side. The stoichiometry of the subunits, gp10 and gp11, of this complex was determined in combination with sedimentation equilibrium, Edman degradation of the complex and sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE). From the results of Edman degradation and SDS-PAGE, the molar ratio of gp10 and gp11 was approximately 1. On the other hand, the molecular weight of the purified gp10-gp11 complex was determined by sedimentation equilibrium to be 284000+/-7000, which is in good agreement with the expected value of 269840 if the stoichiometry is 3:3. Furthermore, comparison of the results in the presence and in the absence of reducing reagent, 2-mercaptoethanol (2-ME), in SDS-PAGE revealed that two molecules of gp10 in the complex formed a disulfide bond, while the third gp10 molecule does not participate in the disulfide bond formation.