A virally induced T-cell leukemia-lymphoma developed spontaneously in 80% of inbred AKR mice between the ages of 6 and 12 months. Advanced cancer inhibited macrophage accumulation at inflammatory sites, with the degree of inhibition being directly related to the elevation in peripheral white blood cell (WBC) count and inversely correlated with the weight of the tumorous lymph nodes and thymus. Sixty AKR mice were examined biweekly between the ages of 5 1/2 and 10 months; macrophage inflammatory responses were compared with peripheral WBC counts. Depressed macrophage responses were not observed before onset of leukemia but did occur coincidently with or within 2 weeks of each episode of leukemia. The conclusion was reached that an inhibition of macrophage function typically occurred with the onset of spontaneous leukemia and during the terminal phase of illness, but this inhibition did not precede leukemia, was probably not related to lymph node or thymus enlargement, and was not always present despite persistent tumor growth.