PK11195 cotreatments significantly increase the AML growth-inhibiting effects of subcurative GO doses in NOD/SCID mice bearing HL-60 xenografts. HL-60 tumors substantially increased in size across the 24-day experimental period, and GO treatments reduced tumor growth relative to vehicle controls, but this effect was not significant. PK11195 alone had little effect on HL-60 tumor growth, but tumor growth was significantly less in mice treated with PK11195 plus GO than in vehicle controls on days +13 (P < .01), +17 (P < .05), +21 (P < .05), and +24 (P < .01), and significantly less than in GO-treated mice on days +13 (P < .01), +17 (P < .05), and +21 (P < .05), as denoted. Data are shown as means ± SEM for 5 to 6 mice per treatment group.