The existence and morphology of pores of the order of 150 A, in radius, within collagen fibrils is delineated. Structural features of collagen from air-dried kangaroo tail tendon, impreO"nated with mercury at a p ress ure of 10,800 psi., are compared with those of fibril s not. exposed to mercury. A helical configuration of sub fibrillar elements is suggested by t~e electron microgaphs of individual fibrils. The possible use of mercury under hydrostattc pressure as a technique in staini ng and preserving the structur e of biological materials for electron microscopy is advanced. These findings constitute a .visual c