It was the purpose of this study to examine whether the number of bacteria in the nose and nasopharynx changes during a common cold. Samples for bacteriological culture were taken from the nasal cavity of 29 and from the nasopharynx of 26 adult patients with naturally acquired colds. The bacteriological samples were taken on days 2, 4, 8 16, 32 and 64 after the first nasal symptoms. The results showed that there were few pathogenic bacteria in the nasal cavity and nasopharynx. There was no tendency to an increase in the number of positive cultures during the cold, although most patients had macroscopically purulent discharge on days 3-5. In conclusion, the study indicates that during an uncomplicated common cold, a bacterial infection is not responsible for the occurrence of purulent nasal discharge which may be a direct consequence of the viral infection.