Certain strains of Penicillium roqueforti and of Phoma exigua var. inoxydabilis synthesize eremophilane-like sesquiterpenes, some of which are biologically active. We have chosen a group of these compounds which are related chemically in order to study the relationships between their chemical structure and their biological properties (toxicity in vivo, capacity of inhibiting RNA and protein syntheses in eucaryotes). This work concerns PR toxin and 2 derived metabolites synthesized by P. roqueforti and phenomenone which is produced by P. exigua. The results allow us to discuss the problem of the active sites of these compounds.