An Agrobacterium tumefaciens potato disc assay was used to screen certain natural products (25 µg/disc) from fungi for crown gall tumor/antitumor induction. Monorden (-75.0%), cladosporin (-79.0%), monocillin IV (-79.6%), duclauxin (-96.0%), diplodiol (-96.3%), and chaetoglobosin K (-99.0%) displayed concentration-dependent responses at 5, 10, 25, and 50 µg/disc. These natural products were not antimicrobial as determined by sensitivity tests using fungi and bacteria, inclusive of A. tumefaciens. Fungi, and other living organisms, constitute a vast resource of natural products that may have varied agricultural, environmental, and pharmaceutical applications. Several fungal metabolites are promising antitumor agents. An example is duclauxin, produced by several Penicillium species (1-3), which is effective against numerous tumor cell types including Ehrlich's ascites carcinoma, lymphadenoma L-5178, HeLa, P-388, and murine leukemia L-1210 (2,4,5). The mode of action of duclauxin involves inhibition of mitochondrial respiration (prevents ATP synthesis) (2,4-6). Crown gall tumors on potato discs are induced by Ti plasmids in Agrobacterium tumefaciens and are well correlated with animal tumor cell assays as a predictor of tumor/antitumor induction, as developed by McLaughlin and his coworkers (7-9). In this study we demonstrate that duclauxin and several other natural products isolated from fungi are effective against crown-gall tumor induction. Except for duclauxin, the natural products screened are either not commercially available or have had limited prior testing as potential antitumor agents.