The therapeutic efficacy of Ceftriaxone and Cefonicid 1 g I.M. single-dose against S. Pneumoniae, H. Influenzae and K. Pneumoniae in tonsil and lung infections was studied using bactericidal quotient (BQ). Samples of lung tissue and serum were obtained from two groups of surgical patients, treated with Ceftriaxone or Cefonicid 1 g I.M., 2-8-16 and 24 hours before surgery. From two other groups of 10 surgical patients, treated as above, samples of tonsil tissue and serum were obtained. In lung tissue the higher levels of Ceftriaxone and Cefonicid appeared at the second hour (11.54 +/- 1.59 mcg/g and 11.4 +/- 2.49 mcg/g respectively); the lower values were observed at the 24th hour (2.18 +/- 0.47 mcg/g for Ceftriaxone and 0.64 +/- 0.21 mcg/g for Cefonicid). Higher Ceftriaxone and Cefonicid levels also appeared in tonsil tissue at the 2nd hour (11.12 +/- 2.42 mcg/g and 9.22 +/- 3.12 mcg/g respectively); the lower values were observed at the 24th hour (1.54 +/- 0.46 mcg/g for Ceftriaxone and 0.42 +/- 0.23 mcg/g for Cefonicid). BQ values were estimated using the ratio between the mean concentrations of Ceftriaxone and Cefonicid in tissue samples and the MBC mean values determined for the above mentioned pathogens, isolated in hospital. Ceftriaxone always showed BQ values greater than 1 during 24 hours versus levels observed for the three reported pathogens. Cefonicid showed BQ values greater than 1 during 24 hours against S. Pneumoniae and BQ values less than 1 for 6-8 hours against H. Influenzae and for 1-4 hours against K. Pneumoniae.