The in vitro activity of Compound BL-S786 was compared with that of cephalothin against 5,762 clinical isolates by the microdilution broth method. BL-S786 demonstrated a broader spectrum and a significantly lower MIC against the Enterobacteriaceae. Although greater susceptibility to BL-S786 than to cephalothin was exhibited by Serratia marcescens, Proteus morganii and Proteus vulgaris, these three species were generally resistant to both drugs. By contrast, the staphylococci were significantly more susceptible to cephalothin than to BL-S786. Resistance to both drugs was demonstrated by Pseudomonas aeruginosa and other pseudomonads, enterococci and Bacteroides fragilis.