The ordinary effusion (``oven'') slit of a conventional molecular beam apparatus was replaced by a slit of special design facing into a supersonic jet from a miniature nozzle. Using ammonia as a test gas, it was observed that at inlet pressures above about 170 mm the molecular beam intensity began to rise far more rapidly than the pressure. The maximum observed intensity exceeded by more than a factor of twenty that obtainable from the effusion slits under optimum conditions and continued to rise rapidly at the highest inlet pressures possible with the available equipment. Certain imperfections of the present apparatus restricted observations to ammonia; their presence indicates that still greater enhancement of molecular beam intensity is possible with an apparatus of refined design.