OBJECTIVE We evaluated by immunohistochemistry HSP70 expression in reactive lymph nodes since its morphological expression and location have not been previously described and correlated with lymphocyte kinetics. RESULTS Ninety-six cases of non-specific lymphadenitis were immunostained for HSP70, CD20, CD3, Ki67, Bcl-2, CD21. The type and the location of HSP70-positive cells were determined. Their number out of 2000 cells in each germinal centre and in each paracortical area was counted at 60x magnification with the help of a quantitative grid. Seventeen percent of germinal centre cells and 7.6% of the paracortex cells were positive. This difference was highly significant. The positively reacting cells were B-cells and had a blast (centroblast or immunoblast) morphology, with negative mantle and marginal lymphocytes and T-cells. Lymphoplasmacytoid cells and plasma cells reacted only weakly or were negative. Germinal centre antigen-presenting cells and interdigitating dendritic cells reacted from lightly to moderately. CONCLUSIONS HSP70 was selectively over-expressed by B-blasts mainly located within germinal centres with a lower number in the paracortex. The difference in the mean number between the two sites was statistically highly significant. No correlation was found with bcl-2 and Ki67 expression. Mantle, marginal and T-lymphocytes were always negative. The biological meaning and role of this over-expression in centroblasts and immunoblasts remain to be elucidated.