Fundamental and clinical studies were carried out on ceftazidime ( CAZ ), a newly synthesized cephalosporin C antibiotic ( CEPs ). The antibacterial activity of CAZ was compared with those of CER, CEZ, CMZ and CPZ against clinical isolates of S. aureus. S. pyogenes. E. coli, K. pneumoniae and P. mirabilis, and with those of GM and CFS against P. aeruginosa. Against S. aureus, the antibacterial activity of CER was highest, followed by that of CEZ. The peak MIC after inoculation of 100-fold dilution was 0.10 microgram/ml with CER and 0.78 microgram/ml with CEZ. But in view of the peak MIC of 6.25 micrograms/ml, the antibacterial activity of CAZ was inferior to that of CPZ by about 2 tubes. This was not surprising, because CAZ was one of the antibiotics in the fifth group of CEPs . The CEPs in the fifth group naturally show high antibacterial activity against S. pyogenes. CAZ , as expected, inhibited the growth of all the strains at the concentration of 0.10 microgram/ml at the inoculation of 100-fold dilution. In the gut bacterial flora such as E. coli, K. pneumoniae and P. mirabilis, CAZ showed the results almost equal to those of other CEPs in the fifth group; the peak MICs of CAZ were 0.20 approximately 0.39, 0.20 approximately 0.39, 0.10 microgram/ml, respectively, at the inoculation of 100-fold dilution, which was good results. In P. mirabilis with the undiluted inoculation, the result of CAZ was slightly inferior to those of the other CEPs in the fifth group previously reported; however, CAZ was prone to be affected by inoculum size, and with the inoculation of 100-fold dilution, MIC of CAZ turned to be as low as 0.10 microgram/ml. Against P. aeruginosa, CAZ showed the activity comparable to that of CFS, the antibiotic considered to have the highest antibacterial activity of all CEPs used in Japan. This finding is in accordance with the findings reported by other authors. The peak MICs of CAZ were 3.13, 12.5 microgram/ml at the inoculation of undiluted solution, and from 1.56 to 3.13 microgram/ml at the inoculation of 100-fold dilution, which were the results equal to, or even better than those of GM. The change in blood levels of CAZ was studied by one shot intravenous injection and 1 hour intravenous drip infusion.(ABSTRACT TRUNCATED AT 400 WORDS)