We reported that the tumor necrosis factor-alpha (TNF-alpha) expression and apoptotic death of oligodendrocytes appeared to be a major pathogenesis of the demyelination of spinal cords of Wistar-King-Aptekman-Hokudai (WKAH) rats with human T lymphocyte virus type I (HTLV-I) infection, HAM rats. In the present study, we examined the sensitivity to TNF-alpha-induced cell death of in vitro-separated oligodendrocytes from HTLV-I-infected WKAH rats. Although the number of non-viable oligodendrocytes increased by adding recombinant TNF-alpha, in a dose-dependent manner, in both HTLV-I-infected and uninfected control rats, oligodendrocytes from the infected rats were more susceptible to TNF-alpha. In situ detection of DNA fragmentation showed apoptotic death of oligodendrocytes. The expression of bcl-2, an anti-apoptotic gene, was strongly down-regulated in oligodendrocytes of the infected rats but not in the control rats. We suggest that the down-regulation of bcl-2 expression in the oligodendrocytes of the HTLV-I-infected rats may increase the susceptibility to TNF-alpha-induced apoptosis of oligodendrocytes, the result being development of HTLV-I-induced myeloneuropathy in rats.