Combination antifungal therapy was assessed in an immunosuppressed rabbit model of invasive aspergillosis. Treatment with fluconazole, amphotericin B, or a combination of both significantly prolonged survival of animals lethally challenged with Aspergillus fumigatus. High-dose amphotericin B was the most effective therapy for invasive aspergillosis. Although no antagonism was seen when fluconazole was given prophylactically or therapeutically in combination with amphotericin B, combination therapy did not augment the antifungal activity of amphotericin B. Animals given a sublethal challenge of A. fumigatus had lower mortality rates when given amphotericin B, fluconazole as treatment or prophylaxis, or various combination therapies. Only animals treated with flucytosine had mortality rates comparable to those of controls. No antagonism was observed with combinations of fluconazole and amphotericin B, flucytosine and amphotericin B, or fluconazole and flucytosine. These observations provide evidence that fluconazole, flucytosine, and amphotericin B used in various combinations are not antagonistic and may provide some insight into the treatment of invasive aspergillosis in humans.