The invention relates to the preparation of the disclosed catalyst system and to the homo- or copolymerization of ethylene with the aid of this catalyst system. The catalyst system contains a catalyst A and a cocatalyst B which, system is suitable for the homopolymerization of ethylene or copolymerization of ethylene with one or more .alpha.-olefins having 3-12 carbon atoms and optionally one or more non-conjugated dienes. Catalyst A contains one or more organomagnesium compounds, one or more transition metal compounds, and one or more halides that do not react spontaneously with the organomagnesium compound(s). The catalyst system also contains at least one transfer agent.