New antibiotics spirocardins A and B were isolated from the culture broth of an actinomycete isolated from a soil sample collected near Lake Hibara, Fukushima Prefecture, Japan. The producing strain was classified as Nocardia sp. SANK 64282. The antibiotics were isolated from the culture filtrate by solvent extraction and purified further by silica gel and preparative reverse phase column chromatography. They were primarily active against Gram-positive bacteria including methicillin-resistant Staphylococcus aureus and limited species of Gram-negative bacteria such as Bacteroides fragilis and Klebsiella pneumoniae. They were also moderately active against several species of Mycoplasma. The molecular formulae of spirocardins A and B were C20H30O6 and C20H32O6, respectively. From their physico-chemical characteristics they were revealed to be diterpenoid antibiotics with closely related structures and the former was easily converted to the latter by the reduction with NaBH4.