Medermycin, a new antibiotic active against gram-positive bacteria including multiple-drug-resistant staphylococci, was isolated from the cultured broths of Streptomyces K73—a strain recently obtained from Tokyo soil and classified as Streptomyces tanashiensis. The strain was cultured at 28°C with aeration and agitation in a medium containing starch (3.0%), peptone (0.5%), meat extract (0.5%), dry yeast (0.3%), and NaCl (1.0%), with maximal medermycin potency (assayed against Staphylococcus aureus FDA 209P) achieved at ~38 hours. Purification involved acid filtration (pH 2.0, Celite 545), Amberlite XAD II adsorption, solvent extraction (chloroform, ethyl acetate), and phosphonomethyl cellulose chromatography, yielding orange crystals after recrystallization from n-butanol. Physicochemical properties included a pKa of 9.5, melting point of 180°C (decomp.), [α]²² 170° (methanol), solubility in water/lower alcohols, quinone group color reactions, UV absorption peaks at 215, 253, and 269 nm (ethanol), IR peaks for quinones (1620, 1645, 1660 cm⁻¹), and a molecular formula of C₂₄H₂₉NO₈·HCl. Antimicrobial testing showed strong activity against gram-positive bacteria (MIC 0.2–0.8 mcg/ml for Staphylococcus aureus strains, including multiple-drug-resistant isolates) but weak activity against gram-negative bacteria. Medermycin exhibited very weak antitumor activity and acute toxicity in mice (LD50: 8.8 mg/kg i.v., 7.8 mg/kg i.p., 620 mg/kg oral). Comparative analyses revealed similarity to luteomycin and antitumor substance No. 289 in chemical characteristics but differences in IR spectra and antitumor activity, suggesting they are structural analogs.