The authors compared the in vitro antifungal activity of itraconazole to that of miconazole against various species of Candida. The MIC of 88 strains of Candida (C. albicans, C. guilliermondii, C. parapsilosis, C. glabrata) was determined by the agar dilution method using casitone medium at pH 5.5-5.6 for miconazole and pH 7.2-7.4 for itraconazole. C. albicans and C. glabrata were found to be less sensitive in vitro to itraconazole than to miconazole; the contrary was observed for C. parapsilosis. The sensitivity of C. guilliermondii to these antifungal agents was similar. The MIC 50, MIC 90 and G.MIC values for itraconazole with respect to 42 strains of C. albicans was 0.10, 1.4 and 0.138 micrograms ml-1, respectively; the MIC 90 and G.MIC values for miconazole were 0.5 and 0.021 micrograms ml-1, respectively. The effect of itraconazole on the ultrastructure of C. albicans yeast cells and spheroplasts was studied by scanning electron microscopy. This triazole molecule modified the cell wall of C. albicans, caused cell stretching and provoked defective separation between mother and daughter cells. Itraconazole altered the cytoplasmic membrane of the spheroplasts causing them to have a "spongy" appearance. Yeast cells treated with itraconazole appeared to liberate their spheroplast with difficulty.