Summary. A number of novel honeycomb structures were encountered among collagen bundles in the capsule lumen of the Golgi tendon organs in the adult Chinese hamster. They varied in size and shape and consisted of a hexagonal aggregation of columnar components (or units), usually perpendicularly disposed with respect to the long axis of the tendon organ. They measured 90-100nm along their inner diameter and less than 1.0am in length, separated by partitions of electron dense materials about 30nm wide. Each columnar unit consisted of a central, globular, electron dense material (20-30nm in diameter) with a constant periodicity of 30-40nm and many (16-20) radially-arranged filaments (about 6nm in diameter), showing a flowerlike pattern. The honeycomb structures were attached to or fused with collagen fibrils or fibers at their margins, and were also attached to the microfibrils and basal laminae of the sensory endings, Schwann cells, and the innermost cell layer of the outer capsule. These findings suggest that the honeycomb structures might contribute to a complicated three-dimensional connection of collagen bundles as a whole, and that this connection might efficiently cause the mechanical deformation of the sensory endings by their squeezing in muscular contractions or passive stretching.