In the course of screening studies for antifungal antibiotics, a new antibiotic, prumycin was isolated from a cultured broth of a Streptomyces sp. strain No. F-1028, which was obtained from a soil sample collected at Kagawa Prefecture, Japan, in 1969. Prumycin is a water soluble, basic substance with a molecular formula of C8H19N3O5. The antibiotic is active against some bacteria and fungi, especially Sclerotinia sp. and Botorytis sp. The following medium was used for production of the antibiotic: glucose 2.0%, peptone 0.5%, meat extract 0.5%, sodium chloride 0.5%, dry yeast 0.3%, and calcium carbonate 0.3%. The pH of the medium was adjusted to 7.0 with sodium hydroxide before sterilization. Antimicrobial activity reached a maximum after 45 hours in a jar fermentor. The antifungal activity was determined by the paper disc plate method using Sclerotinia cinerea as a test organism. The culture filtrate was passed through a column of Amberlite IRC-50 (Na+) and the column was washed with water. The antibiotic was then eluted with 1 N hydrochloric acid and the active eluate concentrated under reduced pressure. After purification by column chromatography on active carbon, using distilled water as a developer, the crude sample was applied to a Sephadex G-10 column and the column developed with distilled water. The active fraction was concentrated in vacuo and acetone was added to yield a white powder. The active substance was then crystallized from methanol to form colorless needles or plates of prumycin hydrochloride: m.p. ca. 195°C (dec), [α]D +63.2° (c 1, H2O). The UV spectrum showed only an end absorption; the IR spectrum measured on a KBr tablet is shown in Fig. 1. Based on elemental analysis and the potentiometric titration method (pKa1: 7.02, pKa2: 8.16 equivalent weight: 122.5), the molecular formula of prumycin was proposed as C8H19N3O5. Prumycin is readily soluble in water, soluble in methanol and dimethyl sulfoxide, slightly soluble in ethanol, and insoluble in most organic solvents. It gives positive color reactions with ninhydrin, Elson-Morgan, Tollens, Benedict and Ehrlich reagents, but negative reactions with Molisch, Sakaguchi, maltol and ferric chloride reagents. The IR spectrum and color reactions indicate that prumycin belongs to the aminosugar group of antibiotics. Prumycin was very stable in acidic solution, but unstable in alkaline. As is shown in Table 1, prumycin was principally active against phytopathogenic fungi such as Sclerotinia sclerotiorum and Botorytis fabae, but inactive against most bacteria and yeasts. The oral administration of 500mg/kg prumycin in mice caused no toxic reaction. Nojirimycin and ezomycin have antimicrobial activities similar to that found for prumycin. Prumycin differs however from these known antibiotics in elemental analyses, UV spectrum, IR spectrum and color reactions. Consequently, prumycin is proposed as a new antibiotic.