Secretion of proinflammatory mediators by activated macrophages plays an important role in the immune response to Trypanosoma cruzi. We have previously reported that AgC10, a glycosylphosphatidylinositol-anchored mucin from T. cruzi, inhibits TNF secretion by activated macrophages (de Diego, J., Punzon, C., Duarte, M. and Fresno, M., Alteration of macrophage function bya Trypanosoma cruzi membrane mucin. J. Immunol. 1997. 159: 4983–4989). In this report we have further investigated the molecular mechanisms underlying this inhibition. AgC10 inhibited TNF, IL-10 and cyclooxygenase-2 (COX-2) synthesis by macrophages activated with LPS or LPS plus IFN-γ in a dose-dependent manner. AgC10 did not affect other aspects of macrophage activation induced by LPS, such as inducible nitric oxide synthase (iNOS) expression. AgC10 also had no effect on TNF or COX-2 transcription or the induction of their promoters but inhibited the stability of TNF and COX-2 mRNA, which are regulated post-transcriptionally by the mitogen-activated protein kinase (MAPK) p38 pathway. AgC10 was found to inhibit both the activation and the activity of p38 MAPK, since MAPK activated protein kinase-2 (MAPKAP-K2 or MK-2) phosphorylation was also strongly inhibited. This led to TNF and COX-2 mRNA destabilization. In contrast, AgC10 did not affect p38 activation induced by TNF. Furthermore, AgC10 inhibition must lie upstream in the MAPK activation pathway by LPS, since this mucin also inhibited extracellularly regulated kinase (ERK) and Jun kinase (JNK)activation.