Dimethyl sulfoxide (DMSO) is commonly used as a solvent for antifungal drugs. Earlier the author has reported the inhibitory effect of DMSO on the growth of many strains of dermatophytes' colonies in dermasel agar and proposed that this could cause the variations between results of different studies for the evaluation of the activities of antifungal drugs. In studies regarding the determination of the effect of antifungal drugs on the germination of arthrospores of dermatophytes it was observed that relatively higher concentrations of DMSO were being used as a solvent for the antifungal drugs, the final concentration in the media being 5%. Therefore, the present study was aimed at determining the effect of different concentrations of DMSO (1.25 to 10%) on the growth of germ tubes of arthrospores of Trichophyton mentagrophytes and Candida albicans, in glucose peptone broth. With DMSO 10% there was a negligible growth of germ tubes of both the arthrospores and yeast; between 2.5 and 7.5% there was a rather linear dose-related inhibitory effect; whereas 1.25% had insignificant effect from controls. The present study shows that besides other factors, variations in the results of the susceptibility tests of antifungal drugs might occur due to the effect of DMSO on the growth of fungi and differences in the final concentration of DMSO in the medium.