The influence of the nature and concentration of the organoaluminium compound during polymerization of ethylene on titanium-magnesium catalysts has been studied. It was established that the type of organoaluminium compound influences the MMD of PE. The dependence of the rate of polymerization of ethylene on the concentrations of Al( iso-Bu) 2H and hydrogen indicate the profound influence of the steric factors on complexing between the catalytic centres and the organoaluminium compound. It was found that the character of the dependences of the mean MMs and polydispersity of PE on the concentration of Al( iso-Bu) 2H is determined by the hydrogen concentration in the reaction zone. From this it is postulated that hydrogen takes part in the chemical modification of the active centres.