Adult CBA mice at 0, 50, 73, and 90 percent levels of inbreeding were used to study variation's in chromosome number from bone marrow cells. Chromosome counts were made from 20 animals at each level of inbreeding except at the 73 percent level where only 13 animals were available. The chromosome number was determined for 24 countable cells per animal at each level, and used to determine if there were significant differences in the variation of chromosome number among the various levels of inbreeding. A modal class of 40N chromosomes was noted for the mice in all groups. However, there was considerable variation within each group. The average percentages of abnormal counts were 23.5 for the controls, 37.9 for the 50 percent inbred level, 28.3 for the 73 percent inbred level and 27.7 for the 90 percent inbred level. An analysis of variance indicated a significant difference between the various levels of inbreeding for the percentage of normal cells. Duncan's Multiple Range Test indicated that the major coutribution to the significant F-ratio came from the 50 percent level of inbreeding. Consequently, the relationship is not linear, but the higher level of abnormal cells occurred at or below the 50 percent level of inbreeding with a decreasing number of abnormal cells at the higher levels. / Master of Science