SUMMARYThis paper describes the electron microscope and gives details of its performance at an operating voltage of 3 MV.Some initial results are reported concerning: penetrating power; resolving power; studies of radiation damage, and other interactions between electrons and crystalline materials. Several observations are also described concerning biological materials (bacteria, frog skeletal muscle, filaments from mushroom spores …).Because at high voltage, radiation damage is considerably reduced, the results obtained indicate a new range of applications of high voltage electron microscopy in biological ultrastructure researc