Abstract: The present invention relates to a novel method of preparing diastereoisomers derived from tetrahydrofolic acid. By reacting (i) (6RS)-folinic acid or one of its salts, or (ii) 5,10-methenyl-5,6,7,8-tetrahydrofolic acid, with HCOOH at pH 2.0-2.6, a precipitate is obtained which consists of a mixture of two diastereoisomers of orthoamide structure and of the formulae ##STR1## in which R is the p-benzoyl-(L)-glutamic acid residue, in a molar ratio IV/III of about 7/3, compound III being of so-called natural configuration and compound IV being of so-called unnatural configuration. The pure compound IV is isolated from said precipitate and the pure compound III is isolated from the corresponding filtrate. The diastereoisomers of the reduced folates and their salts of natural and unnatural configuration are prepared from compounds III and respectively IV.