The antimicrobial activities of cefquinome and tilmicosin were determined in vitro against P. multocida and O. rhinotracheale of poultry origin by serial broth macrodilution. Both antibiotics exhibited good in vitro antibacterial activity against the strains tested. The minimum inhibitory concentration (MIC) for cefquinome was 1.562 mug/ml and for tilmicosin was 0.78 mug/ml for both microorganisms. To investigate the effects of a combination of cefquinome and tilmicosin, the fractional inhibitory concentration (FIC) index was determined by checkerboard assay. The antimicrobial combination was additive against P. multocida (FIC=1) and indifferent against O. rhinotracheale (FIC=2).