The strategy of combining antifungal drugs in a treatment regimen may improve the outcome of invasive candidiasis. Using a well-validated pharmacodynamic murine model of invasive candidiasis, we defined the effect of the combination of amphotericin B deoxycholate (AmB) and 5-fluorocytosine (5FC) by use of the Greco model of drug interaction. The combination was additive, meaning that the experimental effect did not deviate in a statistically significant manner from the null reference model (or additive surface) of the combined effect. From a clinical perspective, the addition of 5FC to a regimen of AmB may enable the near-maximum effect to be reached in circumstances in which the administration of a given dose of AmB alone produces a submaximum effect but an increase in the dose is not possible, because of dose-related toxicity. Our methods provide a way in which some of the complex issues surrounding antifungal combination treatment can be addressed.