C57 BL/6N mice injected intracranially with the A59 strain of mouse hepatitis virus exhibit extensive viral replication in glial cells of the spinal cord and develop demyelinating lesions followed by virus clearing and remyelination. To study how different glial cell types are affected by the disease process, we combine three-color immunofluorescence labeling with tritiated thymidine autoradiography on 1-micron frozen sections of spinal cord. We use three different glial cell specific antibodies (a) to 2',3' cyclic-nucleotide 3' phosphohydrolase (CNP) expressed by oligodendrocytes, (b) to glial fibrillary acidic protein (GFAP) expressed by astrocytes, and (c) the O4 antibody which binds to O-2A progenitor cells in the rat. These progenitor cells, which give rise to oligodendrocytes and type 2 astrocytes and react with the O4 antibody in the adult central nervous system, were present but rare in the spinal cord of uninfected mice. In contrast, cells with the O-2A progenitor phenotype (O4 + only) were increased in number at one week post viral inoculation (1 WPI) and were the only immunostained cells labeled at that time by a 2-h in vivo pulse of tritiated thymidine. Both GFAP+ only and GFAP+, O4+ astrocytes were also increased in the spinal cord at 1 WPI. Between two and four WPI, the infected spinal cord was characterized by the loss of (CNP+, O4+) oligodendrocytes within demyelinating lesions and the presence of O-2A progenitor cells and O4+, GFAP+ astrocytes, both of which could be labeled with thymidine. As remyelination proceeded, CNP immunostaining returned to near normal and tritiated thymidine injected previously during the demyelinating phase now appeared in CNP+ oligodendrocytes. Thus O4 positive O-2A progenitor cells proliferate early in the course of the demyelinating disease, while CNP positive oligodendrocytes do not. The timing of events suggests that the O-2A progenitors may give rise to new oligodendrocytes and to type 2 astrocytes, both of which are likely to be instrumental in the remyelination process.