This work is intended to assess the possibilities of using coal-derived liquids (CDL) represented by a specific type (SRC II) and shale-derived distillate fuel in blends of petroleum-derived fuels in medium-speed, high-output, heavy-duty diesel engines. Conclusions are as follows: (1) Blends of solvent refined coal and diesel fuel may be handled safely by experienced diesel engine mechanics. (2) A serious corrosion problem was found in the fuel pump parts when operating with solvent refined coal blended with petroleum. It is expected that a metallurgy change can overcome this problem. (3) Proper selection of materials for the fuel system is required to permit handling coal-derived liquid fuels. (4) A medium speed, high horsepower, 4-cycle diesel engine can be operated on blends of solvent refined coal and petroleum without serious consequences save the fuel system corrosion previously mentioned. This is based on a single, short durability test. (5) As represented by the product evaluated, 100% shale-derived distillate fuel may be used in a medium speed, high horsepower, 4-cycle diesel engine without significant consequences. (6) The shale product evaluated may be blended with petroleum distillate or petroleum residual materials and used as a fuel for medium speed, high horsepower, 4-cycle diesel engines. 7 references, 24 figures, 20 tables.