A protozoan Tetrahymena pyriformis was a test organism useful in the preliminary screening for antiprotozoals, especially antitrichomonas agents. By this screening method, ikarugamycin, a new antibiotic with specific antiprotozoal activity, was found in the culture of Streptomyces sp. No. 8603 isolated from a soil sample. The morphological characteristics of Streptomyces sp. No. 8603 include formation of gray aerial mycelium, dark brown growth, and aerial hyphae forming a long spiral. Strain No. 8603, belonging to a chromogenic type, was identified as a variety of Streptomyces phaeochromogenes and given the name Streptomyces phaeochromogenes var. ikaruganensis Sakai. Ikarugamycin was isolated as white crystalline needles, decomposing at 252-255°C, and exhibiting optical rotation [α]D0 + 360° (c 1.10, dimethylformamide). Its molecular formula C29H38O4N2 was determined by elementary and mass spectrum analyses. The ultraviolet absorption spectrum showed two maximal peaks at 220 mμ and 325 mμ in methanol. Ikarugamycin showed strong antiprotozoal activities: MIC 0.3~1.25 mcg/ml against Trichomonas vaginalis, MIC 1.0 mcg/ml against Tetrahymena pyriformis W, and MIC 2~10 mcg/ml against Entamoeba histolytica. The median lethal dose (LD50) of ikarugamycin was 6 mg/kg, determined in mice by intraperitoneal administration. Hexahydro-ikarugamycin was obtained by catalytic hydrogenation of ikarugamycin, and its physicochemical and biological properties were investigated. It is similarly active against T. vaginalis as the antitrichomonas agent azalomycin F, and its acute toxicity (LD50 300 mg/kg, ip, in mice) is less than that of ikarugamycin and azalomycin F. In our laboratories, a screening method using Tetrahymena pyriformis as an indicator organism for antiprotozoal products was devised. As a result of this screening program, Streptomyces No. 8603 was found to produce a new antibiotic with strong antiprotozoal activity, designated ikarugamycin. This paper deals with the screening method, taxonomic studies on the producing organism, production and isolation of ikarugamycin, and the physicochemical and biological properties of ikarugamycin. In addition, a low-toxicity derivative of ikarugamycin, hexahydro-ikarugamycin, is presented.