The purpose of our work was to compare sensitivity of autotrophic and heterotrophic organisms used in bioassay to lanthanum and cerium under conditions of a model experiment with aqueous media. Using bioassay methods, we determined pre-lethal effects of La and Ce in heterotrophic Paramecium caudatum and Escherichia coli, as well as autotrophic Chlorella vulgaris and Nostoc linckia. Model solutions of La2(SO4)3·8H2O and Ce2(SO4)3·8H2O were tested in the concentration range of 0.1-200 mg/l. As a result it was shown that heterotrophic organisms are more sensitive to water pollution with La and Ce than autotrophic ones. According to the totality of experiments, cerium turned out to be more toxic than lanthanum. When planning environmental studies of wastewater or reservoirs polluted with REE, we recommend to focus on comparative sensitivity of bioassay methods, taking into account the test-functions used: bioassay for chemotaxis of P. caudatum > bioassay for changes in bioluminescence of E. coli (strain M-17) > bioassay for the content of chlorophyll а and intensity of lipid peroxidation in N. linckia > bioassay on the increase in the number of Cl. vulgaris.