Increased susceptibility of human fetal astrocytes to human T-lymphotropic virus type I in culture. Human T-lymphotropic virus type I (HTLV-I) has been considered as an agent responsible for tropical spastic paraparesis and HTLV-I associated myelopathy. However, the pathogenesis of the diseases remains unclear. In a previous study we demonstrated that HTLV-I could infect adult human astrocytes and oligodendrocytes in vitro, although the rates of infected cells were low, at a rate of 0.1% and 0.01-0.05% respectively. Since mother-to-child transmission has been proposed as one of the major pathways for the prevalence of HTLV-I endemic, in the present study we investigated the susceptibility of human fetal astrocytes to HTLV-I in culture. After two days of co-culturing fetal brain cells with irradiated MT-2 cells (an HTLV-I-producing T-cell line), immunofluorescence staining revealed many positive astrocytes for HTLV-I p19 antigen. Multinucleated giant cells doubly immunoreactive to glial fibrillary acidic protein and HTLV-I antigen were frequently observed and showed a characteristic feature of hairy or fluffy external appearance. The percentage of infected astrocytes became as high as 19.4% at Day 21 of co-culture and then decreased. Electron microscopic examination revealed type C virus-like particles in astrocytes. These results indicate that human fetal astrocytes are more susceptible to HTLV-I infection than adult human astrocytes in tissue culture. 