The stucture of the human lympl vessel wall was studied with a special reference to the muscular architecture, and also to the regional and individual differences. Twenty-eight preserved cadavers of Japanese adults and one fresh corpse were used. The specimens were prepared from them within the range macroscopically possible of obtaining necessary materials. Therefore, the smallest vessels were not involved in the present study. Besides ordinary histological procedures, film specimen method and graphical reconstruction were also applied. 1. The muscle bundles of the lymph vessel wall, ever changing their course, branching out, and anastomosing with one another, form a flat network (muscle bundle group or muscle plate). At the thicker portion, the muscular coat consists of multiple muscle plates. Their bundles intersect with one another obliquely and form diamond meshes, but there never interlace with one another like textile fabrics. Often a group of muscle bundles branches out from one muscle plate and form new musucle plate, the bundles of which are directed in another direction. Such communications of msucle bundles can be observed commonly in between each layer of the muscular coats (see below). 2. Elastica interna of the lymph vessel is poorly developed. It spreads out around the bundels of the inner longitudinal muscle, forming the elastic sheathes. Therefore, it is not necessarily of a single layer, but is often made up of a rather wide zone including longitudinal muscle bundles. 3. The muscles of the intima and the adventitia are thin, generally consisting of a single or double muscle plates running approximately longitudinally. The muscular coat of the media is thick. The muscle bundles rung enerally longitudinally in the inner layer and circularly in the outher layer with ones in transitional state in between. However, some of the vessels show all the muscle bundles of the media running longitudinally, while in others all circularly. The longitudinal muscle bundles of the intima and inner layer of the media, connecting with each other, constitute the inner longitudinal muscle of the lymph vessel. The course and obiqueness of the muscle bundles of the media are extremely irregular so that the coat offers variable patterns. 4. The regional differences in the structure of the lymph vessel are most markedly exhibited in the amount of the longitudinal and circular muscles. Furthermore, systematic differences are involved to a great extent in them, and local ones seem to be less significant. 5. Systematic differences. a) In the most peripheral lymph vessels the wall is thin, and contains only a small quantity of longitudinal muscle, suggesting the transformation to muscleless vessels. However, more proximally the wall becomes fairly thicker, the vessel showing considerable contraction. In this type the muscular coat is composed mostly of longitudinal bundles and only a few circular bundles can be recognized. Still nearer to the center the vessels are now made up of three muscular coats, namely, the inner logitudinal, the middle ciruclar and the outer longitudinal. As compared with the lumen the wall in rather thin, and contains less muscles, the contraction of the vessels growing weaker. According to these differences in the muscular architecture, author divided lymph vessels into five types. Of them the vessels consisted of three muscle coats seem to be designated as conducting lymph ducts. The thoracic duct and other lymph trunks as weel as vessels in the upper and lower limbs (excepting those in the periphery) and the chain of vessels ascending from the deep subinguinal nodes to the lumbar trunks belong to this category. Most of other medium-sized vessels belong to the vessels consisted only of longitudinal muscle.