Temperature elevation constitutes a beneficial component of host defense against viral pathogens. However, virally infected cells may exhibit altered cellular tolerance to thermal stress. Additionally, increased viral oncoprotein expression in virally-infected cells during thermal stress may contribute to cellular neoplastic transformation. Down-regulation of cellular HSP27/28 is inversely associated with the oncogenicity of adenovirus-transformed cells (Zantema et al., 1989). We investigated the effect of thermal stress on the replication of human T-cell lymphotropic leukemia/lymphoma virus type I (HTLV-I) and the expression of cellular heat shock protein 27/28 (HSP27/28) in MT-2 cells, an HTLV-I-transformed human lymphoid cell line. We found that viability of MT-2 cells was reduced by 75% over control levels after three-hour thermal stress. Normal peripheral blood mononuclear cell (PBMC) viability was reduced by only 9% (p {dollar}<{dollar} 0.01). Metabolic ({dollar}\sp3{dollar}H) leucine labelling and immunoblotting of MT-2 cells showed a decrease of HSP27/28 expression during five hours of thermal stress. Northern blotting also demonstrated a decrease of HSP27/28 mRNA transcription. However, the production of HSP27/28 in PBMCs was increased. Thermal stress results in a rapid increase in the phosphorylated form of HSP27/28 in both MT-2 and uninfected lymphoid CEM cells during one-hour thermal stress. The expression of the HSP70 protein family increased during thermal stress in both MT-2 cells and PBM cells. HTLV-I replication in MT-2 cells was markedly increased as detected by viral p24 antigen expression and reverse transcriptase (RT) activity during five hours of thermal stress at 42{dollar}\sp\circ{dollar}C. There were strong negative correlations between the reduced production of cellular HSP27/28 and the increased production of HTLV-I p24 (r = {dollar}-{dollar}0.9390, p {dollar}<{dollar} 0.01), and RT activity (r = {dollar}-{dollar}0.9429, p {dollar}<{dollar} 0.01) in MT-2 cells during five-hour thermal stress. The results suggest that (1) the down-regulation of cellular HSP27/28 in MT-2 cells during thermal stress is associated with increased HTLV-I proliferation, and (2) early posttranslational phosphorylation of HSP27/28 could be a determinant of the ability of MT-2 cells to survive hyperthermia.