CI-920 (PD 110,161) and two analogues (PD 113,270 and PD 113,271) are novel antitumor compounds produced by a new actinomycete characterized as Streptomyces pulveraceus subsp. fostreus ATCC 31906. The antitumor compounds are predominantly produced during the stationary (idiophase) growth phase of the organism. CI-920 is active versus the murine P388 lymphocytic and L1210 lymphoid leukemia with T/C values of 246 and 207, respectively. This compound has no significant antimicrobial activity. In our screening program for antitumor compounds, a fermentation broth of an actinomycete (isolate WP-426) was observed to inhibit the growth of L1210 murine leukemia cells in tissue culture. The biological activity was subsequently found to be due to a complex of at least three antitumor compounds: CI-920 (component A), PD 113,270 (component B), and PD 113,271 (component T). The isolation and characterization of these phosphorus-containing compounds are described in the following paper. In this paper, the taxonomy of the producing organism, fermentation, and biological properties of the compounds are presented.