Cephalothin, a semisynthetic derivative of cephalosporanic acid, was used successfully to treat 77 of 80 infections. Therapy was successful againstStaphylococcus aureus, Streptococcus, Diplococcus pneumoniae, Escherichia coli, Clostridium perfringens, Aerobacter aerogenes, mixedProteus mirabilis, Str faecalis, and mixedStaph aureusand group AStr pyogenesinfections. Miliary tuberculosis,Hemophilus influenzaemeningitis, and vaccinia failed to respond. No cross-sensitivity between cephalothin and penicillin could be documented, suggesting that cephalothin is an effective parenteral antibacterial agent for infections produced by penicillin G-sensitive and resistant staphylococci, pneumococci, streptococci, possiblyC1 perfringens, and selected gram-negative enteric bacteria.