Cephalothin and cephaloridine concentrations of 10 μg/ml inhibited all of 100 Shigella strains in vitro. An additional 60 strains were tested with cephaloridine, cephaloglycin, cephalexin and ampicillin. One strain was resistant to all drugs and another was resistant to ampicillin (MIC 40 /μg/ml) but susceptible to the other drugs. The remaining 58 strains were inhibited by 5 «g/ml or less cephaloridine and ampicillin and by 10μg/ml or less cephaloglycin and cephalexin. A pilot s./tudy of cephaloglycin oral suspension treatment of 9 infants and children with acute shigellosis demonstrated that this antibiotic was no better than placebo therapy by all clinical and bacteriological parameters. Quantitative Shigella fecal counts were reduced slightly from pretreatment levels of 106 to 108/g feces to 103 to 107 during cephaloglycin therapy. Eight of 9 patients developed overgrowth of Candida and/or Pseudomonas in throat and stool cultures. Serum cephaloglycin levels were unmeasurable or low in 14 of 16 specimens. The lack of effective serum and tissue levels probably accounts for the failure of cephaloglycin. Although inhibitory concentrations of cephalexin are relatively high, they can be exceeded in serum, so consideration can be given to clinical trials of cephalexin for acute shigello