The structure-activity relationship for 7-arylacetamido cephalosporins has been extended. Modifications of the 7-aryl group led to improvements in microbiological activity against Gram-positive organisms. However, Gram-negative activity was generally much poorer than that of the lead compound 7-[(2-aminothiazol-4-yl)acetamido]-3-chloro-cephalosporanic acid (A). Modifications of the 3-position did not significantly change the microbiological activity or spectrum. Of the compounds selected for mouse protection studies (ED50's), 7-[(benzothien-3-yl)acetamido]-3-chloro cephalosporin and A showed the best per oral to subcutaneous ED50 ratios.