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1. New halogenated 1,4-naphthoquinones were synthesized and together with other known 1,4-naphthoquinones, were screened for antibacterial activity by a turbidimetric method, and for antifungal activity by the diffusion method on agar plates.2. The half-wave potentials and the influence on the oxidative phosphorylation of some of these compounds were determined.3. 2-chloro-3,2'-chloro-ethyl-1,4-naphthoquinone (half-wave potential=-187 mV) was the most active compound, completely inhibiting cell respiration.4. While the natural active naphthoquinones, vitamin K and ubiquinones, possess, as substituent, the electron repelling methyl group, the microbiologically active 1,4-naphthoquinones are substituted, in the quinone moiety, with electron attracting groups such as OH or Cl.5. The half-wave potentials can give only an initial indication of the activity of the compounds studied; a good correlation, on the contrary, can be found between the ultraviolet spectra of such compounds and their activity which seems to depend on the ability of active compounds to exist in an extensively conjugated structure and to form hydrogen bonds.