Grepafloxacin, (S)- is an asymmetric fluoroquinolone derivative which possesses high tissue penetrability as well as strong, broad-spectrum antimicrobial activities. Grepafloxacin has a chiral center and therefore has two optical enantiomeric isomers, R(+)- and S(-)-grepafloxacin. In neutrophil respiratory burst induced by N-formyl-methionyl leucyl-phenylalanine grepafloxacin induces a priming effect. The R(+) enantiomer of grepafloxacin induced a more potent priming effect than did S(-)-grepafloxacin. R(+)-Grepafloxacin also produced a more potent translocation of both p47- and p67-phox proteins to membrane fractions of neutrophils. Grepafloxacin-induced primed superoxide generation was significantly inhibited by pretreatment with PD169316 and SB203580, p38 mitogen-activated protein kinase (MAPK) inhibitors, but not with PD98059, a specific inhibitor of the upstream kinase that activates p44/42 MAPK, or SP600125, an inhibitor of stress-activated protein kinase/c-Jun N-terminal kinase (JNK).