Onychomycosis is a common adult human mycosis, and dermatophytes of the Trichophyton genera are the most common causative agent. Many antimycotic agents are safe and highly effective for the treatment of dermatophytosis, and are available for clinical practice. Successful treatment depends on the ability of antifungal drugs to eradicate the fungal isolates. The aim of this work was to determine the MICs of four antifungal drugs (fluconazole, itraconazole, terbinafine and griseofulvin) recognized for ungual dermatophytosis treatment caused by Trichophyton species, especially Trichophyton mentagrophytes and Trichophyton rubrum. MICs were determined using a broth microdilution method in accordance with Clinical and Laboratory Standards Institute approved standard M38-A with some modifications, such as an incubation temperature of 28 degrees C, an incubation time of 7 days and inocula constituted of only microconidia. The results showed that the activities of terbinafine and itraconazole were significantly higher (MICs of <0.007-0.031 and 0.015-0.25 microg ml(-1), respectively) than other tested agents. All isolates had reduced susceptibility to fluconazole (1-64 microg ml(-1)). The MIC of griseofulvin varied among strains (MICs of 0.062-1 microg ml(-1)). The parameters adopted to perform susceptibility testing of T. rubrum and T. mentagrophytes to antifungal agents appeared to be suitable and reliable, and could contribute to the possible development of a standard protocol.