Two groups of dogs were studied from the initial diagnosis of chronic GVHD to euthanasia. In the control group, dogs, H642, H634, H678, H694, H708, H720, H709, H698, were untreated during this period aside from antibiotic and fluid support. The anti-ICOS mAb-treated group, dogs, H789, H799, H800, H803, H805, received either one or two courses of anti-ICOS mAb. The difference in the survival periods of the two groups was significant (log-rank test).