Background: Chaperonin 10 (Cpn10) is a mitochondrial protein essential for protein folding. In vitro studies have also shown that Cpn10 reduces production of pro-inflammatory cytokines and chemokines, and up-regulates interleukin-10 in response to agonist stimulation of Toll-like receptors. A protective effect for Cpn10 in experimental autoimmune encephalomyelitis has been demonstrated with suppression of inflammatory markers. Here we report the first clinical trials of this agent administered intravenously in human subjects. Methods: Two dose ranging and tolerability studies were undertaken in 16 healthy volunteers (Phase Ia - single doses of 1 mg, 2.5 mg, 5 mg or 10 mg with 1:3 subjects receiving placebo) and 10 patients with multiple sclerosis (Phase Ib - 5 daily doses of 2.5 mg or 5 mg with 1:4 patients receiving placebo) using a double-blind, randomised design. Intravenous administration was over 10 minutes. Adverse events, clinical data, pharmacodynamic and pharmacological parameters were monitored. Results: There were no serious adverse events and no discernible trends observed for other adverse events. The half-life of Cpn10 was approximately one hour and suppression of TNF-a production in a peripheral blood mononuclear cell assay was seen when blood was taken 8-hr post dose. A peak dose pharmacodynamic effect was seen at 5 mg. No significant increase in anti-Cpn10 antibodies was seen. There was no discernable clinical effect in the multiple sclerosis volunteers. Conclusions: Cpn10 is well tolerated when administered intravenously to both healthy and multiple sclerosis affected human subjects. Anticipated modulation of the innate immune system was observed in both healthy and affected subjects.