AS101 (ammonium trichloro(dioxyethylene-0,0')tellurate) is a new synthetic compound previously described by us as having immunomodulating properties and minimal toxicity. Phase II clinical trials are currently in progress with AS101 on cancer patients. AS101 has been recently found to have both radioprotective and chemoprotective effects on hemopoiesis of irradiated mice or mice treated with various chemotherapeutic drugs. The present research was designed to study the in vivo induction of liver acute phase proteins secretion in mice or patients treated with AS101. Induction of these proteins, some of which have the capacity to scavenge free radicals, may contribute to radioprotection. We present evidence that treatment with the immunomodulator AS101 increases production of a variety of acute phase proteins. We demonstrate a significant elevation of serum amyloid A (SAA) in sera of treated mice, as well as an increase in SAA, factor B and ceruloplasmin in sera of patients treated with AS101. The same AS101 treatment was shown to decrease the amount of the negative acute phase protein, albumin. In addition we show that IL-1, IL-6 and TNF-alpha are important mediators of changes in SAA concentrations induced by AS101. Abrogation of SAA production in AS101 treated mice by any one of the anti IL-1R, IL-6R or TNF-alpha antibodies indicates that at least in mice, SAA production is not controlled by a single extracellular signal, but rather it is regulated, at the least, by all three cytokines in various combinations. A better understanding of the mechanism by which AS101 confers radioprotection will enable us to use it more effectively in the restoration of hemopoiesis in patients following radiation or suffering from overdose or accidental radiation.