Abstract: A diagnostic agent for infectious diseases which is capable of distinguishing among different kinds of bacterial species and which allows simple and non-invasive measurement and/or imaging in a short period of time is provided; and a screening method for a therapeutic agent for infectious diseases caused by microorganisms are provided. A diagnostic agent for infectious diseases caused by nitroimidazole susceptible microorganisms, containing an imidazole derivative or a fused imidazole derivative having at least one nitro group on an imidazole ring, or a labeled form thereof as an active ingredient is provided.

Abstract: A medicine or reagent comprising a bipyrazole derivative of the following formula (I), ##STR1## wherein R.sub.1 and R.sub.2 are individually a hydrogen atom, aryl group, alkyl group having 1-5 carbon atoms, or alkoxycarbonylalkyl group having a total of 3-6 carbon atoms, R.sub.3 and R.sub.4 are individually a hydrogen atom, alkyl group having 1-5 carbon atoms, cycloalkyl group having 5-7 carbon atoms, hydroxyalkyl group having 1-3 carbon atoms, benzyl group, naphthyl group, or substituted or unsubstituted phenyl group, as an active component. The medicine is useful for effectively capturing active oxygen and free radicals which cause adult diseases such as cerebral ischemia, heart disease, digestive system disease, and carcinoma, as well as inflammation, and for diagnosing diseases associated with in vivo free radicals.