Ceftriaxone (CTRX), a newly developed antibiotic agent, was administered at 1,000 mg intravenously twice a day for 5 days to 7 healthy volunteers aged 21 to 26, weighing 61 to 79 kg (mean: 67.7 kg). The fecal bacterial flora was examined and the fecal level of CTRX was determined before, on 3 and 5 days after starting the administration, and on 3, 5 and 10 days after terminating the administration. At the same time, various bacteria isolated from the collected fecal specimens were studied for the sensitivity to CTRX, CEZ, CMZ and CTX. Adverse reactions and influence on various clinical laboratory parameters of CTRX were also examined. The results were as follows. Fecal bacterial flora: Enterobacteriaceae was at the level of 10(5) to 10(8) cells/g (mean: 10(7) cells/g) before starting the administration. The bacteria were detected from none of the 7 subjects on 5 days after starting the administration and on 3 days after terminating the administration. On 10 days after terminating the administration, the bacteria were detected in 6 out of 7 subjects at the level of 10(3) to 10(7) cells/g (mean: 10(7) cells/g). The level was the same as before starting the administration as a whole, but it had not been recovered to the initial level in any individual subject. With regard to other Gram-negative bacilli, P. aeruginosa which was isolated from none of the 7 subjects before starting the administration was detected at the level of 10(2) to 10(6) cells/g in 1 or 2 subjects on 3 days after starting, and on 3, 5 and 10 days after terminating the administration. Concerning Gram-positive bacteria, Staphylococcus sp. was isolated at the level of 10(3) to 10(4) cells/g in 3 subjects before starting the administration. The detection rate was decreased after starting the administration; it was detected in 2 and 1 subjects on 3 and 5 days after starting the administration, respectively, and in 1 subject on 3 days after terminating the administration. Then the detection rate was gradually increased; i.e., in 3 and 5 subjects on 5 and 10 days after terminating the administration, respectively. The level became gradually higher from on 5 days after terminating the administration than before starting the administration. Enterococcus sp. was isolated at the level of 10(5) to 10(8) cells/g (mean: 10(7) cells/g) in all of the 7 subjects before starting the administration. It was was detected in 1 and 4 subjects on 3 and 5 days, respectively, after starting the administration.(ABSTRACT TRUNCATED AT 400 WORDS)