Abstract: The invention relates to the use of glycerol ethers as agents that activate the biological effects of at least one substance selected from a herbicide, fungicide or insecticide. The invention essentially relates to the use of glycerol ether as an agent that activates the biological effects of at least one substance selected from a herbicide, or fungicide or insecticide, said glycerol ether having formula (1), wherein R1 represents an alkyl group having between 1 and 18 carbon atoms and R2 represents a hydrogen atom or an alkyl group having between 1 and 18 carbon atoms, preferably a methyl or ethyl group. The invention also relates to a phytosanitary composition containing one such glycerol ether, as well as to a phytosanitary treatment method using said composition.