The 3-aminoxy-(E)-2-methoxyiminopropionyl penicillins 13 and cephalosporins 14 and 15 were synthesized and assayed for their antimicrobial activity on Gram-positive and Gram-negative bacteria whether producers of β-lactamases or otherwise. Compounds 13, 14, and 15 exhibited an activity which was generally lower than that of the corresponding phenylacetyl derivative: penicillin G (4), cephaloram (5), and phenylacetamidodesacetoxycephalosporanic acid (6). Furthermore, the comparison of the minimum inhibitory concentration values of some of the new 2-methoxyimino 3-aminoxypropionyl derivatives 13-15 with those of the corresponding ones 1-3 lacking the 2-methoxyimino substituent showed that the introduction of the 2-methoxyimino group of E configuration on the aminoxypropionamido side chain of 1-3 gives compounds (13-15) which do not generally possess better antimicrobial properties.