Human T-lymphotropic virus type I (HTLV-I) is associated with a neurologic disease, HTLV-I-associated myelopathy-tropical spastic paraparesis, in which both pathological and immunological changes are observed within the central nervous system. The pathogenesis of infection in HTLV-I-associated myopathy-tropical spastic paraparesis is not well understood with respect to the cell tropism of HTLV-I and its relationship to the destruction of neural elements. In this study, neuroblastoma cells were infected with HTLV-I by coculturing with HUT-102 cells to demonstrate that cells of neuronal origin are susceptible to this retroviral infection. HTLV-I infection of the neuroblastoma cells was confirmed by verifying the presence of HTLV-I gp46 surface antigens by flow cytometry and by verifying the presence of HTLV-I pX RNA by Northern (RNA) blotting and in situ hybridization techniques. To determine whether HTLV-I infection could potentially lead to changes in cell surface recognition by the immune system, the infected neuroblastoma cells were analyzed for altered HLA expression. The HTLV-I-infected, cocultured neuroblastoma cells were shown, through cell surface antigen expression and RNA transcripts, to express HLA classes I and II. In contrast, cocultured neuroblastoma cells that did not become infected with HTLV-I expressed only HLA class I. HLA class I expression was enhanced by the cytokines tumor necrosis factor alpha and gamma interferon and in the presence of HUT-102 supernatant. In this system, expression of HLA class I and II molecules appeared to be regulated by different mechanisms. HLA class I expression was probably induced by cytokines present in the HUT-102 supernatant and was not dependent on HTLV-I infection. HLA class II expression required HTLV-I infection of the cells. The observation of HTLV-I infection leading to HLA induction in these neuroblastoma cells provides a possible mechanism for immunologic recognition of infected neuronal cells.