A bioluminescent clinical isolate of Streptococcus pneumoniae was used to test the real-time effects of the oxazolidinone antibiotic, linezolid, on metabolism compared with effects on cell replication. Viable counts and bioluminescence measurements showed that linezolid has little bactericidal effect, which was similar at minimum (6 mg/L), intermediate (13 mg/L) and maximum (20 mg/L) serum concentrations. The post-antibiotic effect, however, was shorter when measured by light output than by viable counts. The results demonstrate that bioluminescence provides a rapid and sensitive means of measuring the effect of antimicrobials on bacterial metabolism, and that the latter recovers earlier than commencement of cell replication after linezolid exposure.