Intracellular penetration, accumulation and disposition of antibacterial agents is crucial for effective treatment of infections caused by intracellular bacteria. Intracellular concentrations and locations of both antibacterials and bacteria remain poorly understood and further research is needed to establish the importance of these concepts. For example, concepts that have been shown to be important outcome predictors when applied to concentrations of antibacterial agents in the serum include: (i) the effect of the peak antibacterial serum concentration to minimum concentration inhibitory to 90% of organisms (MIC90) for bacteria; (ii) the effect of length of time the serum antibacterial concentration remains above the MIC90 curve; or (iii) the area under the MIC90 curve, but above the serum antibacterial concentration curve. Further research should determine whether or not these concepts have important applications in an intracellular environment. Intracellular pathogens have been increasingly contributing to respiratory infections in the community. Therefore, on the basis of intracellular activity against bacteria causing respiratory infections, macrolides are favoured as the most broad spectrum class of oral antibacterial agents available for the treatment of patients with community-acquired respiratory infections.