Activity of a titanium catalyst supported on a bimetallic magnesium-aluminium system, involving a Lewis base [MgCl2(THF)(2)/Al(C2H5)(2)Cl], was studied in ethylene polymerization, and the effect of the catalyst composition on the properties of the final polymer produced was investigated. Analysis and discussion of the findings covering also some part of the kinetic study, resulted in defining the roles for MgCl2 and a Lewis base in the third-generation Ziegler-Natta catalysts. MgCl2 forms a bimetallic complex with an organoaluminium compound, which involves also a Lewis base. Its reaction with TiCl4 yields a very active catalyst wherein the Mg:Ti molar ratio is close to 1. This means that MgCl2 is involved in creating catalytic active sites. The structure of these catalytic sites were suggested. Hence, the role for MgCl2 can be twofold : it is a component of a catalytic active site, and it is a support of Mg:Ti > 5. It was found in the study that the catalytic function of MgCl2 can be maintained while its supporting function can be omitted. Thus, MgCl2 can be substituted for Al2O3 and the catalytic system obtained will have the same activity.