The expression of interleukin-5 (IL-5) and granulocyte-macrophage colony-stimulating factor (GM-CSF) was studied in peripheral blood mononuclear cells (PBMC) from atopic and non-atopic subjects after activation with phorbol 12-myristate 13-acetate (PMA). The levels of IL-5 and GM-CSF mRNA were monitored by quantitative polymerase chain reaction (PCR). IL-5 and GM-CSF mRNA was undetectable in quiescent cells. Following PMA stimulation, some atopic patients showed considerably higher levels of IL-5 and GM-CSF mRNA expression than the non-atopic subjects, and there was a significant correlation between the levels of these two cytokines. It was found that activation of IL-5 expression in PBMC requires protein synthesis as does activation of GM-CSF expression, and that PMA is only required during the first few hours of activation. The kinetics of activation indicated that the level of both mRNA increased over 15 hr and remained constant for another 20 hr. The accumulation of IL-5 mRNA lagged about 3 hr behind GM-CSF mRNA accumulation, suggesting that the expression of these two genes is regulated separately. However, GM-CSF expression was not required for IL-5 activation.