Abstract Incubation of a heat-sensitive strain of (t + ) poliovirus with l -cystine produces a considerable enhancement of its thermal stability at temperatures below 50°. The effect is specific in that d -cystine and a number of other compounds are ineffective. The rate of stabilization is proportional to the concentration of cystine and depends on the temperature with a Q 10 of 2.2. A simple kinetic analysis of the data shows that approximately 10 cystine molecules must become attached to the virus to confer maximal stabilization and intermediate degrees of stabilization also occur. The action of cystine takes place in neutral and alkaline solutions; it is absent at acid pH. It can be prevented by p -chloromercuribenzoate and reversed by thioglycolate. These findings suggest that cystine stabilizes the virus by combining with SH groups of the viral protein and by subsequently producing cross linkages in the protein.