The purpose of this study was to determine the effects of ibuprofen on the ability of liposome-encapsulated muramyl tripeptide phosphatidylethanolamine (L-MTP-PE) to activate human blood monocytes in vitro. We undertook these experiments because the major toxic side-effects following L-MTP-PE infusion, fever and chills, could be prevented when ibuprofen was given orally immediately before L-MTP-PE infusion. It was therefore important to determine whether ibuprofen interfered with the macrophage-activation properties of L-MTP-PE. Peripheral blood monocytes were isolated from normal donors, then incubated with L-MTP-PE in the presence or absence of ibuprofen. The cytotoxic properties of the monocytes were assessed by a radioisotope-release assay against A375 cells. Ibuprofen at dose levels of 40 micrograms/ml suppressed the generation of the cytotoxic phenotype but did not interfere with the killing process once the cells were activated. Interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF alpha) production, as well as the mRNA expression of these cytokines, was suppressed by 40 micrograms/ml ibuprofen. Since IL-1 and TNF play a crucial role in the cytotoxic function of monocytes, these findings may explain the mechanism by which ibuprofen inhibited the generation of the cytotoxic phenotype by L-MTP-PE. By contrast, ibuprofen dose levels up to 10 micrograms/ml had no effect on the generation of monocyte-mediated cytotoxicity by L-MTP-PE and no effect on the production, secretion, or mRNA expression of TNF and IL-1. Therefore, we concluded that if ibuprofen is to be used to control the side-effects of L-MTP-PE, blood levels of up to 10 micrograms/ml are desirable. In two of three patients, we determined that an oral dose of 200 mg given immediately before L-MTP-PE infusion could achieve these desired blood levels.