Chronic viral infections of animals associated with immune complex disease resemble a number of human disorders. At present, on the basis of showing (a) virus, host antiviral antibody, and C3 deposited in a granular pattern along the glomerular capillary wall and in the mesangia and (b) virus-host Ig (presumably antiviral antibody) complexes in the circulation, immune complex disease occurs in chronic murine infections with LCM, LDV, MSV, and ADM. In addition, granular deposits in the glomeruli in Coxsachie B, polyoma, other leukemic viral infections in mice, equine anemia, and hog cholera suggest that immune complex disease also occurs in these infections. In transplacental LCM infections maternal antiviral antibody transferred in utero or via milk induces very early and severe immune complex disease. The severity of immune complex nephritis in mice neonatally or transplacentally infected with LCM virus is directly proportional to the amount of Ig elutable from the kidney and to the proportion of this Ig which reacts with the infecting virus.