In the course of the study on antitumor anthracyclic antibiotics, roseorubicins A and B, new members of the rhodomycin group, were isolated from the cultured broth of Actinomyces roseoviolaceus A 529 (IFO 13081). Roseorubicins are produced as orange-red pigments by the shaking culture of strain A 529 at 28°C for 5 days in a medium containing 4% sucrose, 2.5% soybean meal, 0.1% yeast extract, 0.25% NaCl, 0.32% CaCO₃, 0.0005% CuSO₄·5H₂O, 0.0005% MnCl₂·4H₂O and 0.0005% ZnSO₄·7H₂O, pH 7.4, and can be extracted from the mycelium and the culture filtrate with acetone or chloroform, and separated into two major components with Rf 0.2 and 0.1 by silica gel thin-layer (60 F₂₅₄ Merck Co.) chromatography using chloroform - methanol - acetic acid (80:20:4). Roseorubicins are also produced by Actinomyces violascens A517 (IFO 12920), Act. violarus A530 (IFO 13104), Act. violaceochromogenes A549 (IFO 13100) and Streptomyces purpurascens A519 (IFO 13077) as well as Act. roseoviolaceus. On acid hydrolysis in 0.1 N HCl for 30 minutes at 85°C, roseorubicins A and B gave γ-rhodomycinone (mp 210-218°C, MT at m/e 370, and the same CD spectrum as γ-rhodomycinone indicating 9R and 10R). Thin-layer chromatography of acid hydrolysates with n-butanol - acetic acid - water (4:1:1) indicated that roseorubicin A had three kinds of hexoses, corresponding to rhodosamine, 2-deoxyfucose and rhodinose, and B had only rhodosamine. The ¹³C-NMR spectra (in CDCl₃) of roseorubicins A and B indicated the presence of 54 and 36 carbons, respectively. Thus, the structures of roseorubicins A and B were proposed as shown in Fig. 1. In addition to their antimicrobial activities against Gram-positive bacteria, roseorubicins A and B inhibited the growth of cultured L1210 cells (IC₅₀ at 0.04 mcg/ml of A, and at 0.06 mcg/ml of B on day 2). Fifty percent inhibition concentrations for RNA synthesis in L1210 cultured cells were 0.33 mcg/ml of A and 0.4 mcg/ml of B and those for DNA synthesis were 2.8 mcg/ml of A and 2.15 mcg/ml of B.