The efficacy, safety and usefulness of BMY-28100 for the treatment of bacterial pneumonia were compared with those of cefaclor (hereinafter referred to as CCL) in a double-blind study. The daily dosages were 750 mg for BMY-28100 and 1,500 mg for CCL, divided into 3 administrations daily. These drugs were administered orally for at least 14 days. A total of 172 cases were enrolled in this study. Of these, cases which deviated from the protocols were excluded from evaluations. Thus, clinical efficacy was evaluated in 124 cases, adverse reactions were evaluated in 160 cases, and abnormal laboratory test values were evaluated in 146 cases. The following results were obtained. 1. Efficacy rates ("good" or better responses) in bacterial pneumonia cases as evaluated by the subcommittee were 81.7% (49/60) in the BMY-28100 group and 89.1% (41/46) in the CCL group, thus no significant difference was found between the 2 groups. 2. Efficacy rates ("good" or better responses), as evaluated by investigators, in the same bacterial pneumonia cases which were subjected to the evaluation by the subcommittee were 83.3% (50/60) in the BMY-28100 group and 88.9% (40/45) in the CCL group, thus no significant difference between the 2 groups was found also. 3. Bacteriological response rates in bacterial pneumonia cases were 86.2% (25/29) in the BMY-28100 group and 85.7% (18/21) in the CCL group with no significant difference between the 2 groups. 4. Incidences of subjective/objective clinical adverse symptoms were 3.5% (3/85) in the BMY-28100 group and 1.3% (1/75) in the CCL group, and no significant difference was observed between the 2 groups. No significant difference was also found between the 2 groups in incidences of abnormal laboratory test values, as abnormalities were found in 21.1% (16/76) of the cases in the BMY-28100 group and 25.7% (18/70) in the CCL group. 5. As for overall usefulness of the drug in bacterial pneumonia cases, utility rates ("useful" or better evaluations) as evaluated by the subcommittee were 83.6% (46/55) in the BMY-28100 group and 90.5% (38/42) in the CCL group, and the rates as evaluated by investigators in cases judged as evaluable by the subcommittee were 78.3% (47/60) and 82.2% (37/45), respectively. There were no significant differences between the 2 groups. The utility rates as evaluated by investigators in cases in which diseases were diagnosed as bacterial pneumonia or lung abscess by investigators were 78.3% (47/60) in the BMY-28100 group and 82.2% (37/45) in the CCL group.(ABSTRACT TRUNCATED AT 400 WORDS)