A process to prepare a crystalline beta -lactam compound by the addition of a base to an acidic solution of a beta -lactam compound obtained by the addition of an acid to a solution or suspension of a beta -lactam compound obtained by (a) an acylation reaction of 6-amino-penicillanic acid, 7-amino-cephalosporanic acid, 7-amino-3'-desacetoxycephalosporanic acid or 3-chloro-7-aminodesacetoxydesmethylcephalosporanic acid or derivatives thereof with a mixed-anhydride of the Dane salt of any one of the compound comprising D-phenyl-glycine, D-p-hydroxyphenyl-glycine, D-2(1,4-cyclohexadien-1-yl)glycine or (2-aminothiazol-4-yl)-(2-methoxy-imino-acetic acid) in one or more organic solvents, or (b) dissolving or suspending a crude beta -lactam compound in a solution comprising water and/or one or more organic solvents, wherein, to the solution or suspension of a beta -lactam compound, urea or derivatives thereof and/or amide or derivatives thereof are added.