A kinetic study on the alkaline hydrolysis of cefotaxime at pH 10.5 and 37 degrees C has been carried out by using HPLC and 1H NMR. The main resulting degradation products have been isolated and identified. These include, apart from the well-known deacetylcefotaxime, the exocyclic methylene derivative, the 7-epimer of cefotaxime and the 7-epimer of deacetylcefotaxime. The kinetic constants involved in the process have been determined and according to the experimental results the attack of the hydroxyl group on the ester function bonded to the 3'-carbon is the fastest step in the proposed kinetic scheme. It should be emphasized that the base-catalyzed epimerization of the hydrogen at the 7 position clearly depends on the presence of a good electron-withdrawing group at C(3'). On the other hand, no hydrolysis of the amide at position 7 was detected.