The object of this study is to determine the factors that influence the fleece value, the experimental procedure, necessary to discover these points, and the effect of culling sheep toward fleece improvement. The clean yield per fleece and the grade of wool were thought to be of equal importance in earning a greater return per fleece. In making such a comparison, wool samples were taken from twelve Hampshire ewes and twelve South-Down ewes, of which the wool represented Three-eighth Blood and Half-blood respectively. Concentrations were given to the fact that there might be some difference in the production of clean yield between young ewes and old ewes. The samples were subdivided into two age-groups within each breed of sheep. For convenience, these wool samples, taken from ewes ranging from yearling to four years old, were grouped into “Young Group”, and those samples taken from ewes over four years old were grouped into “Aged Group”. The work done will be reported in detail in later chapters. The most important factors that influence clean yield were staple length and grease fleece weight. When staple length or grease fleece weight increased the clean yield also increased due to: (1) the material increase from the increased staple-length and the increased grease fleece weight; (2) the decrease of shrinkage percentage, which is associated with the increase of staple length. Other factors ranged from moderate importance to non-importance in relation to clean yield. (a) Density of fiber was moderately important in relation to clean yield; (b) Body weight was slightly associated with clean yield; (c) Crimp was rather important in wool uniformity than in clean yield; (d) Fineness is extremely important in wool quality instead of clean yield. Fleeces from aged sheep over four years old were lighter in clean yield than fleeces from young sheep. Finer wool fleeces were much less in clean yield than coarse-wool fleeces. Farm return was in favor of coarser wool per fleece. Major emphasis should be placed on the average record of staple length and grease-fleece weight, less emphasis on body weight, and on crimp, in a culling program of all ewes. Intensive measurement should be done on all characters of wool of all rams. If two ewes produce equal amounts of grease fleece, and equal length of staple, the older one should be culled. / M.S.