Keratin particles impregnated with amorolfine or clotrimazole in serial doubling dilutions (64 to 0.125 µg/ml) were used to evaluate the activities of these agents against 20 isolates each of Trichophyton mentagrophytes and Trichophyton rubrum in a yeast carbon broth medium incorporating Alamar Blue dye. The proposed MIC with keratin impregnation (MIC_K) is defined as the lowest concentration of an agent used to impregnate keratin particles that effects a fluorescence-based fungal growth quotient of 0.05 or less. The conventional colorimetric and visual MICs of amorolfine for the dermatophytes, 0.03 µg/ml for T. mentagrophytes and 0.063 µg/ml for T. rubrum, were approximately half of those of clotrimazole for the same isolates. The superiority of the MIC_Ks of amorolfine for isolates of T. mentagrophytes (2.0 µg/ml; range, 0.5 to 8.0 µg/ml) and T. rubrum (4.0 µg/ml; range, 2.0 to 8.0 µg/ml) over those of clotrimazole (32 µg/ml [range, 8.0 to >64 µg/ml] and 64 µg/ml [range, 16 to >64 µg], respectively) may indicate the strong in vivo antidermatophytic activity of amorolfine as a topical agent. The new antidermatophytic susceptibility testing procedure has potential clinical utility for the in vitro screening of agents for use in the topical treatment of superficial mycoses.