Dermatomycoses are among the world's most common diseases and their incidence has increased over recent years, particularly in immunosuppressed patients. In previous studies, the saponin CAY-1 from cayenne pepper (Capsicum frutescens), has shown antifungal activities against Candida albicans and Aspergillus spp. We therefore studied the in vitro antifungal activity of CAY-1 against non-germinating conidia and hyphae of clinical isolates of the dermatophytes Trichophyton mentagrophytes, T. rubrum, T. tonsurans and Microsporum canis. We used a microdilution method to assess the growth inhibitory activities of CAY-1 against conidia (CLSI document M38-A) and a colorimetric procedure (XTT method) to investigate the metabolic inhibitory activity of CAY-1 against hyphae. The minimal inhibitory concentrations (complete visual growth inhibition) of CAY-1 against non-germinating conidia ranged from 10-20 microg/ml for all dermatophyte isolates included in this investigation. In addition, we found >90% inhibition of hyphal metabolic activity of these same isolates with 10-20 microg/ml of CAY-1. Results indicate that CAY-1 merits further investigation as a potential agent for the treatment of dermatomycoses.