The present invention provides a novel oxazolidinone derivative of the formula (I): wherein Rings A and B are defined as in the specification; X1is a single bond, or a heteroatom-containing group selected from the group consisting of —O—, —S—, —NR2—, —CO—, —CS—, —CONR3—, —NR4CO—, —SONR5—, and NR6SO2—, wherein R2, R3, R4, R5, and R6 are independently hydrogen or lower alkyl, or lower alkylene or lower alkenylene each optionally interrupted by said heteroatom containing group; and R1 is hydrogen, or an organic residue which is able to bind to the 5-position of the oxazolidinone ring in oxazolidinone antimicrobial agents, pharmaceutically acceptable salts and solvates thereof which are useful as an antibacterial agent.