The activities of cefamandole and cefoxitin in vitro were compared with that of cephalot~in against s~aphylococci and gram-negative bacilli. Cephalothin was the mo~t active agent agamst staphylococci. Cefamandole exhibited the greatest activity agamst the Enterobacteriaceae, with the exceptions of Serratia marcescens and indolepositive Proteeae, aga~nst which cefoxitin was the most active antibiotic. The activity of ~ewer cephalosporms that are resistant to the ,β-lactamases of gram-negative bactena renders untenable the status of cephalothin as the class representative of the cephalosporins for susceptibility testing.