A considerable variation is noticed between the different species studiedand even between the strains of the same species, in the assimilation of cholesterol in synthetic media,in presence of different concentrations of bile salts and under anaerobiosis conditions. The obtainedresults show that certain strains of Streptococcus thermophilus and Lactobacillus bulgaricus resist bilesalts and assimilate appreciable cholesterol quantities in their presence. The study of associationsshows that only strains assimilating cholesterol in a pure state remain active when they are put in associations,but there is no additional effect. However, the symbiotic effect between Streptococcus thermophilus andLactobacillus bulgaricus of yogurt, with regard to bile salts, is confirmed. The lactic fermenters ofyogurt (Y2) reduce the levels of total cholesterol, HDL-cholesterol and LDL-cholesterol, in a well-balancedway. In all cases, the assimilated quantity of HDL-cholesterol is lower than that of LDL-cholesterol.Moreover, yogurt Y2 keeps a significant number of bacteria, superior to 10(8) cells ml(-1), and has agood taste 10 days after its production.