Distribution of human (CD45⁺) cells in sheep hematopoietic tissues at 24 and 48 hours after transplantation. Values are expressed as the mean percentage of human cells in each tissue taken from 5 animals per group ± 1 standard error (SE), with the total number of human cells detected in the 4 tissues taken as 100%. Significance values are relative to group 1A (medium control). Values from group 1B (nonblocking anti–VLA-4 antibody) were not significantly different than group 1A.