There is persistent excessive production of a number of pro-inflammatory cytokines in patients with rheumatoid arthritis (RA). A number of experimental therapies have been found to be effective in the treatment of RA, although the effects of these therapies on cytokine production have not been evaluated. One such experimental therapy involves administration of a MoAb to intercellular adhesion molecule-1 (ICAM-1) that has been shown to be clinically beneficial in approximately 60% of the patients, with some patients responding for up to 11 months. The current studies were carried out to determine whether the success of this therapy was associated with changes in mRNA levels for pro-inflammatory and other monocyte-derived cytokines in peripheral blood mononuclear cells (PBMC). Cytokine mRNA levels were assessed in freshly isolated unstimulated PBMC by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) after minimal in vitro manipulation of the cells. Anti-ICAM-1 MoAb administration was followed by an increase in IL-1 beta, IL-8, and tumour-necrosis factor-alpha (TNF-alpha) mRNA detected at 2 or 24 h after the initial infusion in the clinical responders, but no persistent change in these cytokine mRNA levels were observed that correlated with clinical benefit. By contrast, IL-6 mRNA levels declined immediately after the first MoAb infusion and reached pretreatment values variably after the 5 day treatment course. IL-6 mRNA levels remained significantly reduced in patients responding to therapy 1 months after treatment. Fluctuations in monocyte numbers within the PBMC after treatment did not account for the observed changes in cytokine mRNA levels. The results suggest that a decline in IL-6 mRNA levels is a pharmacological action of the MoAb to ICAM-1. Moreover, persistently diminished IL-6 mRNA levels induced by anti-ICAM-1 MoAb might be associated with the long-term benefit of this therapy. In addition, monitoring the activation status of monocytes in circulating PBMC may be useful in predicting response to therapy and warrants further investigation in a larger study population.