Expression of heat-shock proteins (hsp) was analysed in the leukaemic cells of 12 patients with acute myeloid leukaemia (AML) and nine patients with chronic myeloid leukaemia (CML). Using monoclonal antibodies to hsp70, hsp90 and hsp60 (ML30, a mycobacterial antigen with homology to human hsp60), we measured hsp levels by flow cytometry of permeabilized cells. Mononuclear cells from 10 healthy volunteers were also examined. The results demonstrate that hsp expression is significantly increased (P < 0.01) in the circulating cells of patients with AML compared with cells from CML patients, and compared with normal peripheral blood mononuclear cells. This increased pattern of expression was found for all three heat-shock protein families included in this study. Mononuclear cells from leukaemic patients showed a heterogenous pattern of hsp expression, between different patients, between cells from individual patients, and between the different hsp proteins examined. It is possible that hsp expression relates to the differentiation state or proliferative potential of these leukaemic cells.