WE have isolated a new group of ansamycin antibiotics with potent antitumour activity, from a fermentation broth of Nocardia sp. No. C-15003 (N-1) and have named it ansamitocin. Structures of ansamitocin were found to be similar to maytansine and related maytansinoids obtained from plant sources by Kupchan et al.1-4 and Wani et al.5. These comounds have strong antitumour activities, but development of production would be difficult, because plants containing maytansinoids are only harvested in tropical areas and their content in the plants is extremely low. Some attempts have been made to find a maytansinoid-producing microorganism6, but no success has been reported. We report here the microbial production, isolation and structural elucidation of these antibiotics and their antitumour activities against several experimental tumours in mice. WE present here a detailed image showing the distribution of mobile protons in a thin section through a human wrist. The image was produced by nuclear magnetic resonance (NMR) techniques. The image consists of 128 by 128 independent picture elements and has a resolution of about 0.4 mm. For the first time images produced by NMR can be compared in quality to those produced by X-ray tomography.