The growth of MET-1 ATL cells in NOD/SCID mice bearing the MET-1 ATL leukemia was inhibited by HAT and MEDI-507. MET-1 ATL cells were transferred into mice. The groups (15 mice/group) included those receiving PBS, 4 weekly doses of 100 μg MEDI-507, 4 weekly doses of 100 μg HAT, a 4-week combination of 100 μg each of MEDI-507 with HAT, and 6 months of weekly doses of 100 μg MEDI-507. The data represent the mean concentration of human β₂μ in nanograms per milliliter. The animals treated in the 4-week MEDI-507, 4-week HAT, 4-week combination of MEDI-507 with HAT, and 6-month MEDI-507 groups had significantly decreased values of β₂μ when compared with those of the PBS control group (on day 28 P < .0001 and on day 60 P < .0001). Furthermore, the animals receiving MEDI-507 for 6 months had significantly decreased levels of β₂μ when assessed on day 60 compared with those of the mice in the 4-week MEDI-507 treatment group (P < .0175).