Fifteen bits were designed, fabricated, and field tested in an effort to improve Polycrystalline Diamond Compact (PDC) bits for hard formations. Some conclusions are presented regarding design of hard formation bits. The performance is described for the 15 bits tested. Some bit design layouts and photographs are shown. The work is considered to be successful and some excellent results were obtained. Bit No. 9 drilled 5949 feet averaging 33 ft/h and Bit No. 16 drilled 1100 feet of hard rock averaging 8.7 ft/h. Problems that were encountered early were resolved and coring penetration rates ranging from two to six times faster than conventional diamond core bits were achieved. The full-face PDC drill bits showed improved penetration over roller cone bits when drilling medium and hard formations but breakage of the nose cutters occurred when extremely hard formations were encountered. 18 figures.