This review deals mainly with lymphocyte subsets in the human nasal mucosa, and with the common cold. Lymphocytes have recently been characterized in biopsy specimens by an immuno-histochemical method (the avidin-biotin, monoclonal antibody peroxidase technique). In summary, the overall T:B cell ratio was 3:1 and that of T helper cells to T suppressor was 2.5:1; topographical differences within the nasal mucosa were identified. Non-specific defence systems, such as interferon, provide some protection against rhinovirus infection, but most important is the presence of specific antibodies against a given antigenic type of virus. Recent results have suggested that a rhinovirus infection does not cause a marked destruction of the epithelial lining, that it is spotty in the nasal mucosa, and that it is most prominent in the nasopharynx. Increased knowledge about the site of infection and how symptoms are produced is essential for a rational approach to the development of anti-viral therapy.