of the invention generally are of relatively low molecular weight i e having a molecular weight in the range of from 90 to 250 preferably in the range of from 100 to 200 examples of dicarboxylic acids and anhydrides include oxalic acid adipic acid glutaric acid maleic acid maleic anhydride fumaric acid succinic acid succinic anhydride terephthalic acid terephthalic anhydride hexahydroterephthalic acid and hexahydrophthalic anhydride with succinic anhydride being preferred the reaction of the dicarboxylic acid or anhydride with fk506 is carried out in a temperature range of from 5 to 30 degrees celsius preferably from 20 to 25 degrees celsius for a period of about 12 hours or longer in the presence of triethylamine preferably the reaction is carried out at atmospheric temperature and pressure typically the reaction is carried out by first mixing the dicarboxylic acid or anhydride with fk506 and a catalyst such as dimethylaminopyridine in a solvent such as freshly distilled methylene chloride thereafter the triethylamine is added while mixing stirring the aforesaid components of the reaction mixture the amount of dicarboxylic acid or anhydride for the preparation of the fk506 half ester may vary however typically an amount is chosen to provide a ratio of five moles of dicarboxylic acid or anhydride to one mole of fk506 so as to provide a molar excess of the acid or anhydride the amount of fk506 and the amount of triethylamine are selected to provide a ratio of about 1 0 mole of fk506 to about 1 mole of.