SUMMARYChanges in cell-mediated hypersensitivity to liver and histocompatibility antigens have been detected, by means of the leucocyte migration test, during rejection of liver transplants. Of the 10 cases studied after an orthotopic liver graft, five developed acute episodes of rejection which were of varying severity. In these five cases, inhibition of leucocyte migration was found to be coincident with deterioration in liver function tests and with the histological appearances of rejection on liver biopsy. Two cases developed a cholestatic jaundice, probably not attributable to rejection, and inhibition of leucocyte migration was not found. Chronic rejection, which developed in two patients, was associated with inhibition of migration in one but not in the other. The leucocyte migration test is therefore of most value in the diagnosis of acute rejection.