1. The intracellular transport of leukotriene C4 (LTC4) in hematopoietic cells such as human monocytes is controlled by an ATP dependent carrier encoded by the multidrug resistance protein1 (MRPI) gene whose function can be blocked by the compound MK-571. Since LTs play a major role in control of cytokine expression in monocytes, we questioned whether blocking of the MRPI mediated function by MK-571 might affect cytokine production. 2. MK-571 strongly enhanced IL-6 expression at mRNA and protein level in lipopolysaccharide (LPS) and interleukin-1 (IL-1) stimulated human monocytes giving rise to 2.0+/-0.4 (x+/-s.d.) and 5.7+/-3.5 fold induction of IL-6 protein secretion. The increase in IL-6 secretion was accompanied by an enhanced phosphorylation of p38 but not of c-Jun-N terminal kinase. 3. The involvement of the kinase signalling pathways was further analysed by using SB203580 and PD98059, specific inhibitors of the p38 and ERK1/2 signalling route. MK-571 mediated upregulation of IL-6 in the presence of IL-1 was partially attenuated by SB203580 and PD98059. Electrophoretic mobility shift assays demonstrated that MK-571 did not affect the IL-1 induced DNA binding activity of Activator Protein-1 and Nuclear Factor-kappaB but rather enhanced the transactivational activity of an IL-6 promoter construct. Finally it was shown that the MK-571 mediated effects on IL-6 secretion could not be inhibited by the LT synthesis inhibitor SB203347 or by the anti-oxidant pyrrolidine dithiocarbamate (PDTC). 4. These results indicate that the membrane transporter MRP1 is involved in the regulation of IL-6 expression in activated human peripheral blood monocytes.