The bactericidal quotient (BQ) assessed in the site of infection represents an essential parameter for evaluating the real bactericidal potency of an antibiotic in vivo. The assessment and knowledge of BQ values allow us to set up a more accurate and appropriate antibacterial therapy. The two drugs--ceftriaxone (Rocephin) and imipenem plus cilastatin (Tienam)--that have been taken into consideration in this study, having a similar antibacterial spectrum though with different kinetics, may have the same BQ values in bronchial secretion versus Haemophilus influenzae, Klebsiella pneumoniae and Streptococcus pneumoniae, when administered at different dosages, i.e. ceftriaxone 1 g (i.v.) every 24 h, imipenem 0.5 g (i.v.) every 8 h.