We previously isolated and reported new bioactive metabolites1~4) from clinical isolates of pathogenic Nocardia and our continuing studies on the production of secondary metabolites by pathogenic Nocardia showed that these are limited to Nocardia brasiliensis strains5). Isolates of N. brasiliensis have not been considered taxonomically heterogenous in comparison with those of N. asteroides and N. farcinica, although most studies have not included large numbers of isolates6). Recently, however, RUIMY et al.7) proposed a new taxon among isolates of N. brasiliensis associated with invasive nocardiosis. This new taxon, N. pseudobrasiliensis differs from N. brasiliensis in the hydrolysis of adenine, βlactamase patterns on isoelectric focusing, and specific mycolic acid pattern6), but clinical recognition is difficult because these tests are not routinely used in the clinical laboratory. Simple tests such as antibiotic production for the identification of this species are still required.We recently isolated and reported the new anthracycline antibiotics, nocardicyclins A and B3) (Fig. 1) from a type strain of N. pseudobrasiliensis IFM 0624T. We could not confirm the production of the antibiotics from N. brasiliensis sensu stricto when we tested 84 strains of N. brasiliensis5), and therefore, we were interested if this is a species-specific character of N. pseudobrasiliensis. We therefore examined the production of nocardicyclins by other clinical isolates of N. pseudobrasiliensis. In the current paper we also report on the in vivo antitumor activity of nocardicyclin A against P388 and P388/ADR leukemias in mice.