AbstractThe juice of freshly macerated unripe hulls of the black walnut (Juglans nigra) has been used for many years in folk medicine as a treatment for localized, topical fungal infections such as ringworm. It has been proposed that the biological activity of the walnut hulls is due to the presence of the simple naphthoquinone, juglone (5‐hydroxy‐1,4‐naphthoquinone), which has been isolated from the unripe hulls by sublimation. Since the fresh juice of unripe walnut hulls is utilized for the treatment of ringworm, and since it has been speculated that the activity is due to the presence of juglone, it was of interest to determine the efficacy of juglone as compared to standard commercially available antifungal agents. The comparative efficacy was determined by evaluation of the minimum inhibitory concentration (MIC) values of juglone and the standard antifungal agents clotrimazole, triacetin, tolnaftate, griseofulvin, zinc undecylenate, selenium sulfide as well as two investigational antifungal antibiotics, liriodenine and liriodenine methiodide, for two dermatophytes, Trichophyton mentagrophytes and Microsporum gypseum. MIC values for juglone showed it to have moderate antifungal activity and to be as effective as certain commercially available antifungal agents such as zinc undecylenate and selenium sulfide.