Previous studies in our laboratory have demonstrated that the development of antiviral activity of human leukocyte interferon (IF) in nasal epithelial cells is time and concentration dependent and that the loss of intranasally applied human leukocyte IF is rapid. The present studies compared the activity of IF applied intranasally either by nasal drops or by a saturated cotton pledget. Adult volunteers had IF applied to an area of nasal mucosa (2 by 2 cm(2)) either by repeated nose drops or by a saturated cotton pledget that was applied to the nasal mucosa and left in place for 1 h. Nasal epithelial cells scraped from the area of application, as well as the control, untreated side of the same volunteers, were challenged with vesicular stomatitis virus. No significant reduction in mean virus yield was found in volunteers who received 80,000 U by nose drops. Significant reduction (P < 0.025) in mean virus yield was found in cells obtained 4 h after 80,000, 50,000, or 20,000 U was applied by cotton pledget or in volunteers pretreated with oral antihistamines prior to receiving 80,000 U by nose drops. These experiments indicate that nasal epithelial cells can be made antiviral in vivo by application of human leukocyte IF. However, practical usefulness of human leukocyte IF for prophylaxis against respiratory viral infections may depend on the method of local application.