The goal of this study was to investigate whether administration of interleukin-2 (IL-2) would improve the outcome of cows with malignant catarrhal fever (MCF). The study population consisted of ten healthy control cows and 22 cows with MCF. Nineteen cows with MCF and all of the controls were treated with either 2'500 U IL-2 or 25'000 U IL-2, administered intravenously. Three cows with MCF were not treated with IL-2 (MCF controls). All of the cows with MCF received danofloxacin, flunixin meglumine and intravenous fluid therapy. Blood samples for haematological and biochemical evaluation were collected once daily for six days in all cows. Of the 19 cows treated with IL-2, 13 were eutha nized because of deterioration. All cows with MCF that did not receive IL-2 died. The clinical condition of six cows treated with 2'500 U IL-2 gradually improved. Sur viving cows had significantly higher total leukocyte counts than cows that died or were euthanized. The main reason for leukopenia in non-surviving vs. surviv ing cows was persistent lymphopenia. Use of the lower IL-2 dose was associated with clinical recovery in some cows and this treatment might therefore be considered in valuable cows, provided that the lymphocyte count is within the reference interval.