We have previously reported an increase of human T-lymphotropic leukemia/lymphoma virus type I (HTLV-I) replication after heat treatment of MT-2 cells, an HTLV-I-transformed human lymphoid cell line. In this study, we investigated the effect of heat stress on the expression of Hsp27/28 in MT-2 cells. In contrast with previous studies of other cell types, a decrease of Hsp27/28 expression in MT-2 cells and an increase of Hsp70 family proteins in both MT-2 and uninfected lymphoid CEM cells were found following heat treatment at 42 degrees C. Furthermore, heat treatment resulted in an early rapid increase in the phosphorylated form of Hsp27/28 in both MT-2 and CEM cells. The results suggest that early post-translational phosphorylation of HSP27/28 could be a determinant of the ability of MT-2 cells to survive hyperthermia.