Populations of peripheral blood leukocytes were enumerated in 15 volunteers challenged by intranasal inoculation with rhinovirus serotype 25. The results demonstrated a significant decrease in total lymphocyte count among infected persons on the third day after challenge with the virus (P less than .01). The change in lymphocyte count was associated with a significant decrease in total T cells, as determined by monoclonal antibodies (both T11+ and T3+, P less than .02), but not in B cells (B7+). Among the subsets of T cells, T4+ (T helper/inducer) and T8+ (T suppressor/cytotoxic) lymphocytes both declined in number, but only the change in the T4+ subset was significant. For each of the lymphocyte populations that decreased significantly (T3+, T11+, and T4+) there was a strong correlation with increased severity of symptoms. Persons who had the greatest decrease in total lymphocyte count also shed virus most frequently. The number of nonlymphocyte leukocytes increased with the severity of the symptoms. These data show that T lymphocytes (particularly the T4+ population) are related to both the progression of infection and the symptoms of the rhinovirus common cold.