Various second- and third-generation cephem antibiotics were administered to infants 2 years of age or less. Excluding Streptococcus faecalis, which is resistant to cephems, all of the intestinal bacteria decreased in number, and in many cases these were replaced by yeasts. A positive reaction for protein induced by vitamin K absence or antagonism (PIVKA II) occurred in 25 to 63% of the subjects administered cephems possessing a methylthiotetrazole group, but not in those dosed with cefotaxime or ceftazidime. The effects of cefotaxime and latamoxef (moxalactam) on platelet ADP aggregation were also investigated. When these drugs were administered to clinical patients, moxalactam showed stronger inhibition of aggregation than cefotaxime.