A process for preparing a crystalline, carbapenem monosodium salt of formula IIa: ##STR00001## characterized by the steps a) adding about 10 to 30% of an organic solvent to an aqueous solution of the carbapenem, b) cooling the resulting solution to less than -5.degree. C., c) adjusting the pH using a solution containing an acid in an organic solvent to give the appropriate pH for crystallization, and d) crystallizing the compound by adding methanol, and a C.sub.2-5 alcohol at between -5 and -25.degree. C.