Cilofungin, amphotericin B, and a combination of the two drugs were compared in a model of aspergillosis in immunocompetent mice in three experiments. Cilofungin was equivalent to amphotericin B in preventing death and eradicating cerebral aspergillosis, but it did not sterilize the kidneys. This is the first demonstration of the in vivo activity of cilofungin against any fungus other than Candida albicans. The mortality with combination therapy was higher than those with amphotericin B alone (P = 0.003) and cilofungin alone (P = 0.054), as was weight loss after infection, indicating antagonism between cilofungin and amphotericin B in this model. The mechanisms of action and antagonism remain to be explained.