Miconazole at 10 micrograms ml-1 inhibited the growth of exponential phase cultures of Candida albicans and released intracellular K+. Higher concentration of miconazole were, however, required to cause cell death: at neutral pH, complete killing occurred at 30 micrograms ml-1, while at pH 3.0 or 4.5, there was only partial killing with miconazole up to 80 micrograms ml-1. Efficient killing of C. albicans by miconazole occurred both at low temperature and when cells were incubated in buffer alone. It is proposed that both the fungistatic and fungicidal actions of miconazole are due to its direct interaction with the cellular membranes of C. albicans rather than to an inhibition of biochemical reactions. Divalent cations protected C. albicans from both the fungistatic and fungicidal effects of miconazole and this was probably due to a competition between the ions and miconazole in its positively charged form for negatively charged binding sites. Candida albicans increased in resistance to miconazole-induced K+ release during the stationary phase of a batch culture. Development of this resistance required efficient aeration.