When rats with developing or established adjuvant arthritis are treated with mitoxantrone, the hind paw inflammation associated with the disease is inhibited. Radiographic analysis of the hind paws indicates that the agent suppresses joint destruction associated with the lesion. Comparative studies with cyclophosphamide indicate that mitoxantrone is more efficacious and at least 20 times more potent than cyclophosphamide. Mitoxantrone also appeared more effective when given by the subcutaneous route than the peritoneal route of administration.