Cryptotanshinone and dihydrotanshinone I, constituents of a medicinal plant, Salvia miltiorrhiza Bunge, had antibacterial activity against a broad range of Gram positive bacteria. These compounds generated superoxide radicals in Bacillus subtilis lysates. A recombination-deficient mutant strain of B. subtilis was 2- to 8-fold more sensitive than a wild strain, and this hypersensitivity was reduced in the presence of dithiothreitol as an antioxidant. DNA, RNA, and protein syntheses in B. subtilis were non-selectively inhibited by these compounds. These results suggest that superoxide radicals are important in the antibacterial actions of the agents.