2-Oxazolidinone derivs. of formula (I) and their tautomers, isomers and salts are new. R1 = N3, OH, OR2, OSO2R3 or NR4R5; R2 = 1-8C acyl or a protecting gp.; R3 = 1-4C alkyl or phenyl opt. substd. by 1-4C alkyl; R4 = H, 3-6C cycloalkyl, phenyl, 1-8C alkyl, 1-8C alkoxy or protecting gp.; R5 = as for R4 or COR6, PO(OR7)(OR8) or SO2R9; R6 = 1-8C alkyl, 2-8C alkenyl (both opt. substd. by CN, halo or CF3), H, opt. halo-substd. 3-6C cycloalkyl, CF3, 1-8C alkoxy, phenyl, benzyloxy, 1-6C thioalkyl, acyl, NR10R11 or 5-membered heteroaryl contg. 1-3 heteroatoms (S, N and/or O), opt. substd. by 1-3C alkyl; R10, R11 = H, phenyl or 1-6C alkyl; R7, R8 = H or 1-4C alkyl; R9 = 1-4C alkyl or phenyl; A = a gp. of formula (a); ring A is opt. substd.; X = CO, CS, SO, SO2, P(OH) or P(O)(OH); Y = O, S or NR13; R12, R13 = CONH2, NH2, NHSO2R, SO2NH2 or S(O)fR (all opt. substd.), 3-pyrrolidinyl, 4- piperidinyl (both opt. N-substd. by benzyl), 5-nitro-2-pyridyl, 2- or 4-pyridyl, COCCl3, 1-6C acyl (opt. substd. by CF3, CCl3, OH or (opt. aryl-substd.) 1-6C alkoxy); R = alkyl, phenyl, benzyl or tolyl; f = 0-2.