Anti-apoptotic oncoproteins Bcl-2 and Bcl-xL are overexpressed in many cancers, resulting in multidrug resistance. To develop chemical tools for understanding their regulatory mechanisms, we constructed a cell-based chemical-genetic screening system and isolated incednine (1) from Streptomyces sp. ML694-90F3. Structural elucidation via 2D-NMR, modified Mosher's method, and X-ray crystallography revealed 1 as a novel compound with a 24-membered macrolactam core (containing an enol-ether amide) and two aminosugars. Biological assays showed that incednine overcomes the anti-apoptotic resistance of Bcl-xL/Bcl-2-overexpressing cells to chemotherapeutic agents (e.g., adriamycin) by inhibiting Bcl-2/Bcl-xL function, yet does not block Bcl-xL's binding to pro-apoptotic Bax. Thus, incednine represents a structurally and functionally unique modulator of Bcl-2/Bcl-xL, serving as a valuable tool for studying their anti-apoptotic mechanisms.