A method is disclosed for preparing hydrazides from hydrazine and an acyl chloridewhich comprises the steps of (a) preparing a stirred substantially uniform slurrycomprising hydrazine and an inert solvent at low temperature； and (b) addingan acyl chloride continuously to said slurry. The method avoids or limits productionof undesired bis-hydrazide by-products. The method is used to prepare 3-methyl-3-mercaptobutanoicacid hydrazide, a molecule used to link calicheamicin to a monoclonal antibody.
