Many patients with oral candidiasis respond very slowly or not at all to therapy with amphotericin. Strains of Candida albicans were collected from 17 patients clinically resistant and from 15 who responded to a normal course of amphotericin treatment. Minimal inhibitory concentrations (MIC) determined on diagnostic sensitivity test agar plates gave values of: amphotericin 0-5 mg/l; nystatin 50 i.u./ml; chlorhexidine 12.5 mg/l. No clear MIC could be determined with plates containing miconazole. No difference was noted in MIC values between the 2 groups of patients. Tube-dilution tests in Sabouraud's broth gave MIC values of: amphotericin 0.25 mg/l; nystatin 12.5 i.u./ml; chlorhexidine 1.5 mg/l; miconazole 8-32 mg/l; ketonazole 64 mg/l. Persistence of oral candidiasis is not an indication of infection with resistant organisms. Despite difficulties in in vitro sensitivity testing of miconazole a clinical trial of the drug for treating oral candidiasis is indicated.