Structures of macbecin I (1) and II (2), new antitumor antibiotics isolated from the culture broth of Nocardia sp. No. C-14919, have been elucidated as benzenoid ansamycins on the basis of chemical evidence and spectral analyses. Macbecin I (1) and II (2) are antitumor antibiotics produced by Nocardia sp. No. C-14919, and also show antibacterial, antifungal, and antiprotozoal activities. In this paper, we wish to report the structural elucidation of 1 and 2.