Pleural fluid and serum pharmacokinetics of ceftriaxone were performed in thirteen patients with pleural effusion. One gram of ceftriaxone was administered once daily intravenously in six patients and intramuscularly in seven patients. Ceftriaxone concentrations were measured in serum and pleural fluids in both groups on the first day of administration and in four patients of the intramuscular group on the fourth day of administration. The mean serum peak concentration at 1 h was 199 mg/L (S.E.M. 63.2) in the iv group and 80.5 mg/L (S.E.M. 12.0) in the im group. The mean serum trough concentrations in the two groups at 24 h were 27.5 mg/L (S.E.M. 12.6) and 29.7 mg/L (S.E.M. 5.2) respectively. In the pleural fluid, mean peak concentration was 20.1 mg/L (S.E.M. 4.7) at 6 h in the iv group and 15.3 mg/L (S.E.M. 5.1) at 12 h in the im group. The mean trough concentration was 9.6 mg/L (S.E.M. 1.9) and 13.3 mg/L (S.E.M. 3.1) at 24 h in the two groups respectively. On the fourth day of intramuscular administration the serum and pleural fluid peak and trough concentrations were higher when compared with the first day, consistent with a cumulative effect. The serum and pleural fluid concentrations of ceftriaxone following intravenous and intramuscular administration were well above the MIC90 of most common respiratory pathogens indicating good penetration into extracellular spaces. Further, these serum and pleural fluid antibiotic concentrations could be maintained even after a single intramuscular injection of the drug, thus indicating its usefulness as a parenteral mode of therapy on a domicilliary basis with a significant cost-saving potential. In conclusion, intramuscular administration of ceftriaxone would appear to be a convenient method of administering parenteral therapy in lower respiratory tract infections in the hospital and community, with pharmacokinetics very similar to those exhibited by the intravenous route.