In the course of screening for antitumor agents, seven structurally related active compounds designated as TMC-1 A (1) to G (7) were isolated from an actinomycete strain Streptomyces sp. A-230. Of them, TMC-1 A (1), B (2), C (3) and D (4) were found to be new members of the manumycin class of antibiotics, while the rest TMC-1 E (5), F (6) and G (7) were identified as manumycins D, A and C respectively on the basis of spectral data. Four new antitumor antibiotics TMC-1 A, B, C and D were isolated from the fermentation broth of Streptomyces sp. A-230. Spectroscopic studies have shown that TMC-1A to D were new members of the manumycin class of antibiotics, and these antibiotics showed cytotoxic activities against various tumor cell lines in vitro. This paper describes taxonomy of the producing strain, production, isolation, physico-chemical properties, structure elucidation and biological properties of these four new antibiotics.