Leukocyte populations in the secretions of volunteers challenged by intranasal inoculation with rhinovirus serotype 25 were evaluated by means of flow cytometry. With the light-scatter properties of peripheral blood leukocytes as the standard of reference, significant increases (P less than .05) of both lymphocytes and phagocytes (polymorphonuclear leukocytes plus monocytes) were detected in the nasal secretions of persons infected by the viral challenge. There was a direct correlation of nasopharyngeal symptom severity with both the percentage of lymphocytes (P less than .05) and the percentage of phagocytes (P less than .001). Monoclonal antibodies for specific cell-membrane antigens identified the lymphocytes and phagocytes as leukocytes and also demonstrated the presence of a population of monocytic cells during the phase of maximal symptoms. The panel of monoclonals chosen did not unequivocally identify a lymphocyte population except in the presence of nosebleed. However, the results show that flow cytometry can be used to investigate nasal-secretion cell populations during the rhinovirus common cold.