When iron-molybdenum polyoxometalates (POM) were introduced to rats, the accumulation of heat shock proteins (HSP) in the thymus and spleen were revealed without altering the histological structure of the organs. This finding indicates both the stressing impact of the POM and the protective action of the HSP. Lymphocyte apoptosis increase and H1 histone protein increase are supposed to be indicators preceding the destructive changes in immunopoietic organs.