Short-term stimulation of peripheral blood monocytes (PBMo) and cells of the monocytic cell line MONO-MAC-6 with lipopoly-saccharide (LPS) induces high tumor necrosis factor (TNF)alpha mRNA levels. In contrast to the results obtained with primary cells, this effect could not be inhibited by preincubating the cell line with recombinant human interleukin-4 (rh IL-4). This deficiency in response to the cytokine was not caused by a general unresponsiveness of MONO-MAC-6 cells to IL-4. Thus, the expression of the monocyte-associated differentiation markers CD14 and monocyte-specific esterase (MSE), upregulated by long-term stimulation with LPS, could be decreased by IL-4. Long-term LPS treatment apparently induced IL-4 responsiveness of the cell line. While IL-4R alpha mRNA was upregulated about 3-fold, this positive effect was not apparent at the cell surface protein level. In contrast to the constitutive alpha chain expression, the IL-4R gamma chain expression could not be detected with a specific mAb nor by Northern blot analysis. However, reverse transcriptase polymerase chain reaction (RT-PCR) demonstrated the presence of low-level IL-4R gamma chain mRNA in the cell line. We suggest that the low reactivity of the cells to IL-4 might be correlated with the low expression of the gamma chain.