Assay systems have been developed in order to differentiate between the modes of action of certain bleaching herbicides. These include inhibition of chlorophyll or carotenoid biosynthesis, and initiation of pigment degradation. Herbicidal compounds with phytoene desaturase as their primary target site were investigated in a cell-free carotenogenic system from Aphanocapsa. In a comparative study, the structural prerequisites for inhibition of phytoene desaturase were established for both benzophenone analogs and various m-phenoxybenzamides. This inhibitory action of the latter compounds is enhanced by lipophilic groups with certain steric properties.