The nutritional stabilities of four major dietary folates were studied as their corresponding monoglutamates and were compared to pteroylglutamate (folic acid) itself. The study of the monoglutamyl rather than polyglutamyl forms was justified since the former are formed during the course of digestion and also addition of extra glutamyl residues is unlikely to affect the types of nutritional instability associated with these derivatives. Since ability to support growth in Lactobacillus casei is known to reflect nutritional activity in man this organism was used in the stability studies. It was found that pteroylglutamate and 5-formyltetrahydropteroylglutamate had nutritional stabilities of the order of weeks although the stability of the former was decreased by phosphate. Surprisingly 10-formyltetrahydropteroylglutamate was nutritionally more stable than expected, possibly due to its conversion to the more stable oxidized 10-formylpteroylglutamate or to the reduced 5-formyl derivative. In contrast 5-methyltetrahydropteroylglutamate was much less stable nutritionally than expected.Unsubstituted tetrahydropteroylglutamate was most unstable nutritionally but in contrast to the other derivatives examined it was more stable under acidic than basic conditions. Ascorbate was found to be a far superior stabilizing agent than 2-mercaptoethanol at comparable concentrations.