1. Changes in proton-magnetic-resonance spectra were followed during the reaction of cephalosporins, deacetylcephalosporins, deacetoxycephalosporins and a Δ2-cephalosporin with ND3 in D2O. 2. Changes in proton-magnetic-resonance spectra were also followed during the hydrolysis of a cephalosporin and a deacetylcephalosporin in D2O with a β-lactamase. 3. Structures for the reaction products are proposed. 4. The signals obtained after aminolysis of the β-lactam ring of a cephalosporin indicate that the reaction is accompanied by expulsion of the acetoxy group as acetate, formation of a double bond in the Δ4-position and the appearance of an exocyclic methylene group. 5. Aminolysis of deacetyl- and deacetoxycephalosporins can occur without immediate structural changes in the dihydrothiazine ring. 6. In contrast, the ring structure of the first product of enzymic hydrolysis of a deacetylcephalosporin is apparently identical with that of the product of aminolysis of the cephalosporin itself.
