In the course of a screening program for actinomycetes which produce antibacterial antibiotics, we repeatedly isolated aurodox-type antibiotics which were active against Neisseria caviae ATCC 14659 and inactive against Staphylococcus aureus ATCC6538. These antibiotics, which act upon elongation factor Tu, have a characteristic spectrum of antibacterial activity: they inhibit anaerobes, neisseriae and streptococci, but are ineffective against S. aureus. The producing strains we isolated were mainly Streptomyces spp. occurring with a frequency of about one producer found for every three thousand strains screened. Among the sixteen antibiotics belonging to the aurodox-type class, aurodox, kirromycin, A73A, factumycin and kirrothricin occurred more frequently. We also observed that the streptomycetes co-produced humidin-type antibiotics. These macrolides, which often contaminate preparations of the aurodox-type antibiotics, exhibit high toxicity on both prokaryotic and eukaryotic cells. Humidin-type antibiotics were found mainly adsorbed onto the mycelium of the producing strains and were determined in crude extracts from their UV spectrum and their toxic effect on yeast cells. From a soil sample collected in India, we isolated an Actinoplanes strain, A8924, which produced kirromycin and did not produce any humidin-type antibiotic. Identification of the antibiotic was made using ¹H NMR and LC-MS. Strain A8924 typically had a bright orange vegetative mycelium composed of twisted hyphae with a diameter of 1 ~ 1.5 μm, bearing on the surface multispored spherical sporangia with a diameter of 9~13 μm. The sporophores are straight to slightly curved about 8~15 μm long and 2 μm in diameter. The oval spores, 1.5~2 μm in diameter, are highly motile by means of several flagella arranged in polar tufts. Chemotaxonomic assessment of strain A8924 revealed meso-diaminopimelic acid and glycine as distinguishing components of the cell wall. Xylose and arabinose were the major sugars in the whole cell hydrolysate. According to the classification of Lechevalier and Lechevalier, this is a cell wall type IID. On the basis of both its morphological and chemotaxonomic characteristics, strain A8924 was assigned to the genus Actinoplanes. There are no other examples reported in the literature of a member of genus Actinoplanes producing an antibiotic of the kirromycin-like family.