The lactate dehydrogenase-elevating virus (LDH-virus) (Riley et al. 1960; Riley, 1968) is unusual in a number of ways including its rapid rate of replication (its average replication cycle is approximately 30 min.), the production of high virus titres reaching peak concentrations of 1010 to 1011 ID 50/ml. of blood plasma in 12 to 18 hr, and the rapid decline after 24 to 72 hr of 3 to 4 logarithms of virus infectivity to yield a stable lifelong viraemia of approximately 104 to 106 ID 50/ml. Both the acute and chronic viraemias are clinically benign with no evidence of any tissue damage, loss of weight, decrease in longevity, or cytopathic effects in the host. An understanding of the nature of the unknown inhibitory or virus-limiting conditions responsible for the abrupt decrease in virus concentration and for its continued suppression is obviously pertinent. The possible role of interferon in this process is the subject of this report.