We investigated the serum concentration of interleukin-6 (IL-6) and four IL-6 family cytokines - oncostatin M (OSM), leukaemia inhibitory factor (LIF), interleukin-11 (IL-11) and ciliary neurotrophic factor (CNTF) as well as IL-6 soluble receptor (sIL-6R) - using an enzyme-linked immunosorbent assay (ELISA) in 67 patients with multiple myeloma (MM) and 24 healthy controls, for a possible association between the serum levels of these peptides with disease activity and known prognostic factors. sIL-6R was detectable in all 67 and IL-6 in 65 (97%) patients. Both peptides were measurable in all healthy controls. In contrast, OSM was detectable in 30 (44.8%) MM patients and in only four (16.6%) normal individuals. The serum levels of IL-6, OSM and sIL-6R were significantly higher in MM patients compared with control group (P < 0.001, P < 0.03 and P < 0. 001 respectively). The highest concentrations of these cytokines were found in patients with progressive disease and the lowest in MM patients with stable disease and in healthy persons. LIF was detectable in four (6%), CNTF in 28 (41.8%) and IL-11 in eight (11. 9%) of the patients with MM. In the control group LIF, CNTF and IL-11 were measurable in 8.3%, 33.3% and 8.3% respectively. The serum concentration of these cytokines did not correlate either with clinical stage or with the phase of disease and was similar to those in healthy individuals. We found significant positive correlation between IL-6 levels and OSM (P < 0.001). We also observed positive correlation between beta2-M concentration and serum levels of IL-6 (P < 0.002), sIL-6R (P < 0.02) and OSM (P < 0.04) as well as a positive relationship between CRP and IL-6 (P < 0.001) and OSM (P < 0.002). In conclusion, the serum levels of IL-6, OSM and sIL-6R, but not LIF, IL-11 and CNTF, may be useful markers of MM activity.