OBJECTIVE measurement of baseline soluble interleukin 6 receptor (sIL-6R) and interleukin 6 (IL-6) levels in biological fluids in non-pathological conditions. METHODS Blood and urine were obtained from healthy volunteers. Cerebrospinal fluid (CSF) and synovial fluid (SF) were obtained from patients during spinal puncture and athroscopy, respectively. Only CSF and SF of patients with proven non-pathological conditions were used in this study. Both sIL-6R and IL-6 were measured using ELISAs. It was shown that neither did sIL-6R interfere with the IL-6 ELISA nor did IL-6 interfere in the sIL-6R ELISA. Moreover, addition of recombinant sIL-6R to the IL-6 bio-assay (B9) did not influence IL-6 recovery. RESULTS using our sIL-6R ELISA we found baseline levels for sIL-6R in serum of 76.6 +/- 19.3 ng/ml in serum and 3.7 +/- 1.3 ng/ml in urine. In non-pathological conditions sIL-6R concentrations in CSF are 1.6 +/- 0.4 ng/ml, and in SF 11.6 +/- 3.3 ng/ml, while IL-6 concentrations are below detectable ranges in these fluids. CONCLUSIONS sIL-6R levels are detectable in serum, urine, CSF and SF during non-pathological conditions. sIL-6R levels in serum outrange levels in CSF, urine and SF and large interindividual differences in baseline concentrations for sIL-6R exist.