The in vitro inhibitory activities of cefaclor and cefatrizine, two new orally absorbed cephalosporin antibiotics, against 44 isolates of anaerobic pathogenic bacteria were measured using the agar dilution procedure of the World Health Organization International Collaborative Study. Tests also were performed with cephalexin, cephaloglycin, and cephadrine, as well as with the parenteral cephamycin antibiotic cefoxitin. Cefoxitin was the most active antibiotic and inhibited the majority of isolates at a concentration of ⩽4 μg/ml. None of the oral cephalosporins was clearly superior against all of the anaerobic isolates; only cephadrine and cefatrizine appeared to have any potential clinical value.