Undecylenic acid and tolnaftate were tested in an in vitro test system to evaluate their relative "killing time" efficacy against Trichophyton mentagrophytes, Trichophyton rubrum, and Epidermophyton floccosum. Commercial products containing these active agents were tested similarly. The pure active agents were equivalent in activity. The commercial product containing undecylenic acid appeared to be more effective against the test organisms than did the product containing tolnaftate.