The protective effect of interferon on infection with coxsackievirus type A 16 (CA-16) or enterovirus type 71 (EV-71) in newborn mice was examined. Subcutaneous administration of murine interferon (MuIFN-alpha/beta) into the infected mice produced a protective effect against infection with CA-16 or EV-71. It was found that the time of administration of MuIFN was important in relation to the cycle of infection. Protection was observed when MuIFN was given once daily for several days, from one day before or after infection with the lethal dose of CA-16 or EV-71. These results suggest that interferon may directly suppress infection with CA-16 and not indirectly suppress it by the medium of macrophages, natural killer cells, and T cells.