The common cold is one of the most frequent illnesses caused by viral infection. Recently, we have reported that oral administration of cystine and theanine (CT) to mice enhanced the humoral immune response associated with antibody production. Based on this mouse study, we investigated the effects of CT supplementation on the common cold in humans as a pilot study. A total of 176 healthy male volunteers were randomized to receive either placebo or CT (490 mg) tablets twice daily for 35 days. The incidence outcome was assessed using the definition in our laboratory based on questionnaires regarding cold symptoms. The incidence of subjects with colds during the trial was significantly lower in the CT group than in the placebo group, although the duration of the colds was not significantly different between the groups. These results suggest that CT supplementation may be useful for the prevention of the common cold.