The response of a host animal to primary viral infection is complex, and the factors essential for recovery from the infection are difficult to define. Immune responses, both cell-mediated and hulnoral, interferon production, pyrexia, and local changes in acidity and oxygen tension in infected tissues have all been ascribed possible roles in recovery (1). The importance of any single mechanism probably varies in diseases of different pathogenesis or at particular times during the course of infection, and there will sometimes be interaction and synergism between individual factors.Since neutralizing antibodies and interferon have well documented modes of antiviral activity, they could logically be assigned rather more importance than cell-mediated immunity, which has no clearly demonstrated antiviral mechanism. However, an increasing body of evidence derived from the study of human infants with primary immunological deficiency diseases points to a very important, perhaps even essential role for a thymus-dependent, cellmediated immune response in recovery from vaccinia virus infection (2). Experimental evidence concerning this area of host-virus relations is meager, but recently Hirsch et al. (3) showed that heterologous anti-thymocyte serum (ATS) 1 caused significantly increased mortality and morbidity from intravenous vaccinia infection in mice. They postulated that this was due to suppression of cell-mediated immunity; however no direct evidence was produced to show that the cell-mediated immune response to vaccinia virus was suppressed by ATS. This was assumed on the basis of an immunosuppressive potency test usiag lymphocytic choriomeningitis virus (4).Previously, Allison and Friedman (5, 6) had produced sound evidence that the regression of primary tumours caused by fibroma virus in rabbits was dependent upon cell-mediated immunity. However, this and other work coni Abbreviations use~ in this paper: AMS, rabbit anti-mouse macrophage serum; ATS, rabbit anti-mouse thymocyte serum; MEM, minimal essential medium; NRS, normal rabbit serum; PBS, phosphate-buffered saline; PFU, virus plaque-forming units; RBC, red blood cells WBC, white blood cells.