High activity olefin polymerization catalysts based on organolithium compounds and transition metal compounds are disclosed. In one catalyst system an organolithium compound is employed in combination with titanium tetrachloride and the Li/Ti ratio is in the range of about 20 to about 50. In another catalyst system the organo lithium compound is combined with a rare earth metal halide. For best results these catalyst systems are employed at pressures of at least about 100 psi. In ethylene polymerization an ethylene partial pressure of at least about 100 psi is preferred.
