Antibiotics U 1200 Synthesis and in vitro Activity of New 1β-Methylcarbapenem Derivatives as Antibacterial Agents. -A new series of 1β-methylcarbapenem derivatives having pyrrolidine and piperidine moieties are synthesized and evaluated for their in vitro antibacterial activities. Compound (Ia) shows the most potent antibacterial activities against Gram-positive and Gram-negative bacteria. -(JUNG, M.-H.; HONG, J. H.; CHO, J.-H.; OH*, C.-H.; Arch.