BACKGROUND Allylamines are a newly developed group of drugs possessing a broad spectrum of activity against a wide range of fungi. With the advent of new antifungal drugs, susceptibility testing of fungi is receiving increased attention as important laboratory procedures for aiding in the selection of appropriate drug therapy. METHODS In vitro susceptibility testing of 43 clinical isolates of dermatophytes which included Microsporum sp., (18) Trichophyton sp., (23) and Epidermophyton floccosum (2) were carried out against the two allylamine derivatives, naftifine and terbinafine, by agar dilution and disk diffusion methods. RESULTS Terbinafine was found to be more active minimal inhibitory concentration (MIC range < or = 0.0001-0.1 micrograms/mL), inhibiting 50% (MIC 50) and 90% (MIC 90) of the isolates at 0.01 and 0.1 micrograms/mL, respectively. The MIC 50s and MIC 90s of naftifine were 0.1 micrograms/mL (MIC range 0.001-0.5 micrograms/mL). Both the drugs showed good correlation between the MIC and sizes of zones of inhibition around the disks. Regression analysis was used to measure the degree of correlation between the MIC values and matched averaged zones of inhibition; the correlation coefficients for both terbinafine and naftifine were -0.6841 (P < 0.001) and -0.5455 (P < 0.001), respectively. CONCLUSIONS The allylamines, naftifine and terbinafine, could be used successfully for susceptibility testing of dermatophytes by the disk diffusion method. With proper standardization of the test conditions, in vitro susceptibility testing of filamentous fungi by disk diffusion would become a useful laboratory procedure in the near future for determining the best drug therapy.