Multi-center bacteriological and clinical studies on fosfomycin (FOM) nasal solution were performed in subjects with paranasal sinusitis from January, 1988 to May, 1990. In these studies, we were exclusively responsible for bacterial isolation from clinical sources, bacterial identification and the determination of drug susceptibility. Before local administration of FOM nasal solution, many strains of various bacterial species were isolated from sources totalling 396 cases involved in phase II clinical studies, dose-finding and open clinical studies. From antibacterial activities of FOM against those isolates, we obtained the following conclusions. 1. Among the 447 isolates, Streptococcus spp. occupied 25.7%, Staphylococcus spp. 21.7% and anaerobic Gram-positive cocci (GPC) 13.6%, showing high detection frequency of aerobic and anaerobic GPC. Next to these, Haemophilus influenzae, Pseudomonas aeruginosa, Klebsiella spp. and Branhamella catarrhalis also were often obtained. 2. After exclusion of possibly contaminating strains which might have entered into cultures at samplings or transfers, the MIC50 and the MIC80 of FOM against the remaining 354 isolates were determined to be 12.5 and 25 micrograms/ml, respectively, indicating that local use of FOM would be fully effective to eradicate most of the bacteria. 3. FOM nasal solution showed sufficient eradication efficacy against most clinical isolates of possible causative organisms of paranasal sinusitis, and appeared to be useful as a topical preparation for the treatment of this disease.