The phenotype of cultured fibroblasts from patients affected by Apert's syndrome, a rare connective disorder, differs from that of normal cells in its extracellular matrix macromolecule composition (glycosaminoglycans, collagens and fibronectin) and is further modulated by treatment with interleukins (ILs). As the mechanisms responsible for the changes are unknown, we used our recently described model system for Apert periosteal fibroblasts to ascertain whether the pattern of ILs they secrete into the medium is comparable to that of normal fibroblasts. The results obtained by enzyme-linked immunosorbent assay (ELISA) show that the levels of interleukin-1 (IL-1) and interleukin-6 (IL-6) were lower in Apert than in normal media, whereas levels of IL-1 receptor antagonist (IL-1ra), the natural inhibitor of IL-1, were markedly higher. IL-1 specific bio-activity on thymocyte proliferation was also decreased in Apert supernatants. As we provided also evidence that active transforming growth factor beta (TGFbeta1), an IL-1 antagonist, was not secreted in greater amount in Apert media with respect to normals, the enhancement of IL-1ra appeared critical in down-regulating IL-1. Northern blot analysis of cytokine mRNA revealed no detectable IL-1 or IL-6 gene expression in normal fibroblasts, but high amounts of IL-6 mRNA transcripts in Apert cells. As the increased IL-6 gene expression did not translate into a parallel increase of secreted IL-6, the control of IL-6 secretion may be mainly post-transcriptional. Furthermore, the result that a treatment of the cultures with IL-1ra was able to induce a decrease of IL-6 secretion, suggests that the observed decreased secretion of IL-6 may be due to the autocrine action of overproduction of IL-1ra. The observed imbalance in the production of ILs which we show for the first time suggests ILs may be the natural autocrine regulators of ECM production in Apert fibroblasts. We hypothesize that in vitro differences previously reported in fibroblast phenotypes and several clinical features of Apert's syndrome may correlate with different cytokine patterns.