Combination pairs of the major systematic antimycotic drugs, amphotericin B (AmphB), 5-fluorocytosine (5-FC) and ketoconazole (Ktz) were administered to mice with experimental candidiasis, cryptococcosis and aspergillosis at a variety of combination ratios. The 3 mycoses were produced with 3 strains each of Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus, respectively, which were preselected to represent 3 different degrees of 5-FC sensitivity ('normally sensitive', 'moderately resistant', and 'definitely resistant'). The life-prolonging effect of the combinations was compared with the effect of each partner administered alone at the same and at the double dosage. Using the U test of Mann and Whitney and setting limits which on the whole were more rigorous than those of the isobole methods commonly applied to the study of drug interactions, the effects of the concentrations were classified as 'synergistic', 'additive', 'indifferent' or 'antagonistic'. The combination AmphB plus 5-FC was definitely synergistic or definitely additive in all 3 candidiasis models, the most pronounced synergism occurring in the infection with the 'definitely 5-FC-resistant' C. albicans strain; in cryptococcosis produced by any of the 3 C. neoformans strains the effect was definitely additive, but only slightly additive or indifferent in the 3 aspergillosis models. The combination AmphB plus Ktz was slightly synergistic in candidiasis produced by one C. albicans strain, but definitely antagonistic in this mycosis produced by the remaining 2 strains of the same species; the combination was definitely additive or, even, slightly synergistic in the 3 cryptococcus models, but, again, antagonistic in aspergillosis produced by all 3 strains of A. fumigatus. 5-FC plus Ktz was additive or indifferent in the 3 candidiasis models, but throughout indifferent in cryptococcosis and aspergillosis.