The present study concerns the effect of the experimental diabetogenic encephalomyocarditis (EMC) virus on normal and athymic nude mice of BALB/c origin. The effect of simultaneous immunosuppressive pharmacological treatment with a derivative of cyclophosphamide in a relatively low dose (3 mg/mouse) was also studied. After inoculation with EMC virus, 36% of the normal mice, but none of the nude mice, developed diabetes mellitus and 93% of the normal mice, but none of the nude mice, developed paresis of one or more leg(s). When lower doses of EMC virus were given, few or none of the normal mice developed diabetes or paresis. After treatment with a cyclophosphamide-derivative, the number of paralysed mice increased. EMC virus in abundant amounts could be isolated from the pancreas and heart of all virus-inoculated mice, including the non-diabetic nude mice. Antibodies against EMC virus were found in all groups of virus-inoculated mice, although only in small amounts in nude and immunosuppressed normal mice. Histological examination revealed no significant differences between the islets of Langerhans of the experimental mice, diabetic as well as non-diabetic, and the control mice with respect to lymphocytic infiltration. It is concluded that the thymus-dependent immune system seems to be of decisive importance for the development of diabetes in this virus model.