Subjects with a history of recurrent herpes labialis will be sensitized with either 2% SADBE or placebo. Following this, subjects sensitized with 2% SADBE will received.2% sqauric acid or.5% squaric acid on their cold sore within 72 hours of a recurrence. Subjects sensitized with placebo solution will receive placebo solution on their cold sore within 72 hours of a recurrence. Subjects will be followed for up to 6 mos after application of study medication.