With the development of newer antifungal agents with activity against both yeasts and filamentous fungi, there is an increased need to develop and standardize in vitro assays that will evaluate the activity of antimycotics against filamentous fungi. In vitro analysis of antifungal activity of these agents would also allow for the comparison between different antimycotics, which in turn may clarify the reasons for lack of clinical response or serve as an effective therapy for patients with chronic infection. To determine the in vitro susceptibility of fungal organisms to ciclopirox, terbinafine, ketoconazole and itraconazole and to evaluate the in vitro activity and mode of interaction of ciclopirox in combination with either terbinafine or itraconazole. In the minimum inhibitory concentration (MIC) study 133 strains were evaluated, including dermatophytes (110 strains; 98 from Trichophyton spp.), Candida spp. (14 strains) and nondermatophyte moulds (nine strains). In vitro susceptibility testing was conducted in microbroth dilutions based on the National Committee for Clinical Laboratory Standards (NCCLS) M27-A proposed standard. The testing MIC ranges were 0.003-2 microg mL-1 for ciclopirox and terbinafine, and 0.06-32 microg mL-1 for itraconazole and ketoconazole. For inoculum preparation, dermatophytes were grown on Heinz oatmeal cereal agar slants. Inoculum suspensions of dermatophytes were diluted in RPMI 1640 (Sigma-Aldrich) with the desired final concentration being 2-5 x 103 c.f.u. mL-1. Once inoculated, the microdilution plates were set up according to the NCCLS M27-A method, incubated at 35 degrees C, and read visually following 7 days of incubation. For azole agents, the MIC was the lowest concentration showing 80% growth inhibition; for terbinafine and ciclopirox, the MIC was the lowest concentration showing 100% growth inhibition. In the synergy studies, 29 strains from nondermatophyte species were evaluated using a checkerboard microdilution method. The concentrations tested were: 0 and 0.06-32 microg mL-1 for itraconazole, and 0 and 0.003-4 microg mL-1 for both terbinafine and ciclopirox. Modes of interaction between drugs were classified as synergism, additivism, antagonism or indifference based on fractional inhibitory concentration index values (FIC index). Synergism was defined as an FIC index of or = 2.0. The drug combination was interpreted as indifferent if neither of the drugs had any visible effect on the presence of the other drug. In the MIC study, the dermatophyte MIC values (microg mL-1) (mean +/- SEM) were: ciclopirox (0.04 +/- 0.02), terbinafine (0.04 +/- 0.23), itraconazole (2.28 +/- 7.42) and ketoconazole (0.83 +/- 1.99). The yeast MIC values (microg mL-1) (mean +/- SEM) were: ciclopirox (0.05 +/- 0.02), terbinafine (1.77 +/- 0.58), itraconazole (0.18 +/- 0.27) and ketoconazole (0.56 +/- 0.60). The non-dermatophyte fungi MIC values (microg mL-1) (mean +/- SEM) were: ciclopirox (1.04 +/- 2.62), terbinafine (1.04 +/- 0.95), itraconazole (17.87 +/- 16.75) and ketoconazole (10.69 +/- 13.09). In the synergy study, with ciclopirox in combination with terbinafine, mainly a synergistic or additive reaction was observed; there were no cases of antagonism. For ciclopirox in combination with itraconazole, there were some instances of additivism or synergism, with indifference in the majority of instances; there were no cases of antagonism. In vitro susceptibility testing indicates that ciclopirox may have a broad antimicrobial profile including dermatophytes, yeasts and other nondermatophytes. Terbinafine is extremely potent against dermatophytes. In vitro evaluation of activity of ciclopirox and terbinafine suggests many instances of synergy or additivism; for ciclopirox and itraconazole there may be indifference, synergy or additivism.