In the course of our search for new antibiotics, a novel magnesium-containing antibiotic named magnesidin has been obtained from the cell mass of a pink coloured bacterium. The antibiotic-producing culture was isolated from the washings of the marine alga, Caulerpa peltata, collected from the rocky shores near Bombay, India. Magnesidin is a unique magnesium-containing naturally occurring compound and to the best of our knowledge, is the first antibiotic having this cation. Taxonomically, the bacterium has been identified as a new species of the genus Pseudomonas and has been named Pseudomonas magnesiorubra nov. sp. (ATCC No. 21856), as its morphological and biochemical characteristics differ from all the known species of Pseudomonas described in Bergey's Manual. Fermentations were carried out under submerged culture conditions for 24-30 hours at 28°C in a medium containing 2% glucose, 1.5% peptone, 0.5% yeast extract, 0.5% soluble starch, 3% NaCl and 0.1% MgSO4·7H2O (pH of the medium 7.4). At the end of the fermentation, the cells containing the antibiotic were harvested by centrifugation and the cell mass was repeatedly extracted with hot acetone until the extract was colourless. The orange red extract was decolourised by activated charcoal and the colourless filtrate was concentrated in vacuo to a small volume and chilled. The amorphous crude magnesidin which precipitated out was collected and purified by extraction with ether in a Soxhlet apparatus. The active ether extract was dried and crystallised twice from methanol to give pure magnesidin (yield: 250mg from cell mass obtained from a liter of culture fluid). Magnesidin forms colourless plates, melting point undefined with shrinking and softening from 123~150°C, λmax(methanol) 257nm (E1%m 747.3). It is soluble in ether, ethyl acetate, chloroform, acetic acid, methanol, ethanol, butanol, acetone and pyridine, but insoluble in water. It is very stable even up to 121°C and over a pH range of 2-8. The antibiotic spectrum determined by the serial dilution method is shown in Table 2. The antibiotic magnesidin is effective only against gram-positive bacteria, particularly the spore bearers. It has been found to be toxic (LD50 for mice, 50mg/kg i.p.) and is not absorbed orally. Magnesidin is a mixture of the magnesium salts of two new tetramic acids, 1-acetyl-3-n-hexanoyl-5-ethylidenetetramic acid (n=4) and 1-acetyl-3-n-octanoyl-5-ethylidenetetramic acid (n=6) which are present in ca. 1:1 ratio.