To evaluate the clinical and microbiological efficacy and safety of ceftriaxone in comparison with cefotaxime in treating acute lower respiratory tract infections two open randomized trials were performed. Sixty-two adult hospital in-patients were entered and 58 completed the study. The patients suffered from either acute (broncho-)pneumonia (19pts) or severe exacerbation of chronic bronchopneumonia (39 pts). Forty-one of the patients had severe underlying or concurrent diseases. Diagnoses were in all cases confirmed by isolation of the causative pathogen(s) from bronchial brushing or washing under fiberoptic bronchoscopy. Twenty-eight patients were administered at random with either a single 2g daily dose of ceftriaxone or 2g twice daily dose of cefotaxime (1st trial). Successively (2nd trial) ceftriaxone was administered at a dose of 1g once daily either i.v. or i.m. The duration of treatment ranged from 7 to 12 days. A satisfactory response was observed in all patients suffering from acute pneumonia or bronchopneumonia; the eradication rate of the causative pathogen was 73% and 62% for ceftriaxone and cefotaxime, respectively. Concerning the exacerbation of chronic bronchopneumonia (39 patients) an overall satisfactory response to both treatments was registered in about 80% of cases. No significant differences between the 1g and 2g single daily dosing regimens of ceftriaxone appeared. Both ceftriaxone and cefotaxime were well tolerated: no or minimal changes in laboratory values were noticed. It is concluded that a 1g or 2g single daily dose of ceftriaxone were at least as effective as a 2g twice daily dose of cefotaxime in treating acute lower respiratory tract infections due to susceptible pathogens.