We detected simultaneously JC virus (JCV) antigen and RNA in the frozen brain tissue from a patient with PML using immunocytochemistry and in situ hybridization. Histologically, the majority of JCV antigen positive cells were mainly located at the margin of demyelinated lesions. By simultaneous in situ detection of JCV antigen and RNA, the percentage of both JCV antigen and RNA positive cells in total number cells were 20 in the periplaque region and 2.3 in the central region of plaque respectively. On the contrary, only JCV RNA positive cells were detected 3.6% in the periplaque region and 4.6% in the central region of plaque, while there was not found any positive cells in apparently normal white matter. Moreover, JCV antigen in both antigen and RNA positive cells found in the periplaques was stained homogeneous and filled the whole swollen nucleus, whereas most of both antigen and RNA positive cells appeared in the center of plaques showed small amounts of antigen in the nucleus. From the results, it seems likely that most of the cells displaying only RNA detected in the periplaque were oligodendrocytes in the early stages of productive infection prior to viral protein synthesis. However, a small number of cells displaying only RNA in the center of plaques were likely to be oligodendrocytes and seemed to be in the nonproductive stages of a JCV infection, limited expression of viral protein, influenced by the host immune response including the infiltration of macrophages and humoral immunity.