Four baboons were given a single injection of 5FU (100 mg/kg) and blood counts and circulating clonogenic progenitors were monitored for approximately 3 weeks posttreatment. Two of the animals received between days 5 to 8 a total of four injections of anti-VLA4. Note that in the animals receiving anti-VLA4 there are several days with a significant number of circulating progenitors in contrast with control 5FU animals.