Tumor Necrosis Factor is a pleiotropic inflammatory cytokine. On almost all cells receptors for TNF have been detected. TNF-Receptors (TNF-R) were originally designated as TNF-binding proteins. These TNF-binding proteins present in urine and plasma were later found to be the extra-cellular part of the membrane bound cell signalling receptor for TNF-alpha and TNF-beta. Two types of TNF-Receptors (molecules belonging to the family of nerve growth receptor, CD27, CD40 etc) have been identified: TNF-RI (p55/p60), sCD120a and TNF-RII (p75/p80), sCD120b. The molecules share substantial identity in their extracellular part although the intra-cellular parts are structurally unrelated. This difference is reflected by the difference in function of the two receptors.Activation of various cell types leads to proteolytic cleavage of TNF-R resulting in soluble (extracellular part) TNF-R. Soluble TNF-R (sTNF-R) are considered to play a physiological role in TNF binding and inactivation, although also other functions such as stabilization of TNF in circulation have been mentioned. Detection of sTNF-R offers a means of investigating activation of the immune system both in vitro and in vivo. In plasma of healthy individuals approx. 1 - 4 ng/ml of sTNF-R of each type are present.The ELISA kit is also useful for quantitation of Rhesus monkey and Baboon sTNF-RII in plasma, serum and tissue culture media.