. In this study, we examined the role of mitogen-activated protein (MAP) kinases in the effects of verotoxins (VTs), from Escherichia coli O157:H7, upon both apoptosis and the release of tumour necrosis factor alpha (TNF-alpha) and granulocyte-macrophage colony-stimulated factor (GM-CSF) from human monocytes. 2. Both VT1 and VT2 stimulated a weak, transient increase in c-Jun-N-terminal kinase (JNK) activity and a strong activation of both p38 mitogen-activated protein kinase (MAP kinase) and extracellular-regulated kinase (ERK) activity in human monocytes, which was sustained in the case of p38 MAP kinase. 3. Stimulation of human monocytes with VT2 (100 ng ml-1) did not result in an increase in apoptosis; however, the toxin stimulated the release of both TNF-alpha and GM-CSF. 4. Pretreatment of human monocytes with the p38 MAP kinase inhibitor SB203580, at concentrations from 100 nM to 10 microM, significantly decreased the VT1- and VT2-induced TNF-alpha and GM-CSF release from monocytes. In contrast, inhibition of MEK1 with PD98059 only significantly decreased GM-CSF release. 5. Pretreatment of monocytes with SP600125 inhibited both GM-CSF and TNF-alpha production; however, significant effects upon p38 MAP kinase and ERK activation were observed. 6. Taken together, these results suggest a role for p38 MAP kinase and ERK in cytokine generation in response to the verotoxins. A role for JNK remains undetermined.