The common cold is one of the most frequent illnesses caused by viral infection. A recent study in mice demonstrated that the amino acids cysteine and theanine are associated with enhanced immune activity. The aim of this randomised controlled study was to investigate the effects of cysteine and theanine supplementation on the common cold in humans. A total of 176 healthy male volunteers were divided into two groups and given either supplements containing 700mg of cysteine and 280mg of theanine daily, or a placebo, for 5 weeks. During the trial, the group taking the supplement experienced significantly fewer colds than the placebo group (11.4% Vs 27.1%, p=0.011). The incidence of chills and fever was significantly lower in those taking cysteine and theanine. The average duration of the colds was approximately 10% less in the supplement group than in the placebo group, although this difference was not significant. The authors propose that cysteine and theanine increases glutathione synthesis and antibody production. The results suggest that cysteine and theanine supplementation may be useful for the prevention of the common cold.