The inhibitory activity of SCH 39304 and its enantiomers on radial growth and on the target enzyme, sterol 14alpha-demethylase, in Aspergillus fumigatus was studied to assess the role of stereochemistry in the efficacy of the drug. SCH 39304 and the RR(+) enantiomer were active in inhibiting the growth while no inhibition in the growth was observed with the SS(-) enantiomer. The MIC of SCH 39304 for the growth was about twice that of the RR(+) enantiomer. The differences in IC50s of SCH 39304 and its enantiomers for cell-free ergosterol biosynthesis correlated with their variations in MICs and type II binding spectra indicated the SS(-) enantiomer failed to bind to microsomal P450. These results show that the difference between SS(-) and RR(+) enantiomers in interacting with the target enzyme is the cause for significant difference in the potency between these two forms.