Cefodizime (CDZM), a newly developed injectable cephem antibiotic, was given to 8 healthy male volunteers with ages 23-28 years (mean: 24 years) and body weights 54-73 kg (mean: 64 kg). 4 volunteers received 2 g b.i.d., intravenous infusion for 5 days, and 4 volunteers had 4 g b.i.d. for the same duration. We studied changes in bacterial flora and drug levels in feces, as well as changes in susceptibility of isolated microbes to CDZM, cefmetazole, cefotaxime (CTX) and cefoperazone. Adverse drug reactions and changes in laboratory findings were also investigated. The following results were obtained. 1. In fecal flora for the 2 g b.i.d. group, on day 3 of treatment Enterobacteriaceae were identified in only 2 of the 4 cases. Individual microbe counts decreased or fell below detection limit (10(2) cells/g) on day 3 of treatment. Colony forming units (CFUs) of Enterobacteriaceae on day 5 of treatment were similar to those on day 3. After the end of treatment, its CFUs increased. The mean CFU of Enterobacteriaceae on day 10 after the end of treatment increased to about the same level as before dosing. However, some microbes did not revert to pretreatment levels. Of other Gram-negative bacilli, Pseudomonas sp. increased in one case transiently on day 3 of treatment. After day 3 of treatment, its CFU decreased. Gram-positive microbes showed various changes. Mean CFU of yeast like organism (YLO) increased between day 3 of treatment and day 3 after the end of treatment and its mean CFU decreased on day 10 after the treatment. As for anaerobes, CFUs of Bacteroides fragilis group organisms and overall anaerobes showed no distinctive changes. Clostridium difficile were detected in two cases. In one case, C. difficile was noted at levels between 1.1 X 10(3) cells/g and 2.0 X 10(6) cells/g at the following sampling times: days 1 and 3 of treatment and days 3, 5, 10 and 20 after the end of treatment. In other case, it was noted at levels of 1.8 X 10(5) cells/g on day 20 after the end of treatment. D-1 toxin was detected in one case before dosing, and in three cases (the concentration of D-1 toxin was 250 ng/g or 500 ng/g) on day after treatment. Enterobacteriaceae in fecal flora of 4 cases given 4 g b.i.d. frequently decreased or fell below detection limit on days 3 and 5 of treatment and day 3 after the end of treatment.(ABSTRACT TRUNCATED AT 400 WORDS)