The antibacterial and antimycotic activity of econazole base, an imidazole derivative, was examined in vitro and in experimental infections of mice. Comparative minimal inhibitory concentration (MIC) determinations indicate econazole and miconazole to be of moderate activity against gram-positive bacteria (MICs: 0.78-25 .mu.g/ml) and yeasts (MICs: 1.56-25 .mu.g/ml). Against filamentous fungi, econazole exhibits better in vitro activity than miconazole and, with the exception of Rhizopus oryzae and Absidia corymbifera, MICs are markedly lower than against yeasts. No effect of nutrient media and no effect of the inoculum were observed with the 4 drugs. A strong influence of bovine serum on MIC values suggested a strong protein binding. In experimental candidiasis of mice, no therapeutic effect with econazole base administered orally or i.p. could be observed. In experimental aspergillosis of mice, a slight effect, as demonstrated by the minimum life-prolonging dose of 100 mg/kg, was found. The in vitro and in vivo results are discussed in the light of the available pharmacokinetic and toxicological data. More studies, especially on the pharmacology of econazole and about the clinical efficacy, are needed to come to a definite judgement.