Abstract: Selected polyhydroxyl compounds, comprising monosaccharides, disaccharides and oligosaccharides, as well as certain of their derivatives, are effective in preventing the spontaneous activation of complement in vitro. The effect is enhanced in the presence of an anticoagulant together with a divalent cation chelator. Addition of these compounds makes it possible to store clinical samples at conventional temperatures from -20.degree. to +22.degree. C. for extended periods prior to performing complement protein assays. Effective compounds are selected through a screening protocol which comprises the use of immunoassays for complement fragments together with an algorithm for computing effectiveness. Polyhydroxyl compounds extend similar protection from activation to coagulation proteins.