In this study, we demonstrate that mononuclear cells of human milk have a potential for production of many different cytokines. We applied a technique for cytokine detection at the single-cell level using cytokine specific MAb and immunofluorescence. The characteristic staining pattern obtained represents intracellular cytokine production, which allows for the assessment of the cellular origin of production. Milk mononuclear cells were mitogen-stimulated in vitro and cultured for 4 h and then stained for 13 cytokines. Lipopolysaccharide stimulation induced extensive production of the following monokines: IL-1 alpha, IL-1 beta, IL-1ra, IL-6, IL-8, and tumor necrosis factor-alpha. IL-10 and granulocyte-macrophage colony-stimulating factor were smaller products, although detectable in most samples. The abundant monokine production correlated with the high number of macrophages in milk. Spontaneous monokine production in unstimulated cells could be detected in six out of 11 samples. The highest incidence was evident for IL-8. No spontaneous lymphokine production was detected. Considering the low proportion of lymphocytes, stimulation with phorbol myristate acetate in combination with ionomycin resulted in considerable production of the following lymphokines: IL-2, IL-3, IL-4, IL-10, interferon-gamma, tumor necrosis factor-alpha. Macrophages contributed to the high production of tumor necrosis factor-alpha and GM-CSF. IL-5 synthesis was detectable in only one sample. This work reveals that human milk mononuclear cells are potent producers of cytokines when mitogen stimulated in vitro. The in vivo implications of these findings remain to be investigated further.