In vitro susceptibility of Streptococcus pyogenes, Staphylococcus aureus, Staphylococcus epidermidis, Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli, Serratia marcescens, Hemophilus influenzae, Bacteroides fragilis, and Neisseria gonorrhea to three new second-generation and eight third-generation cephalosporins is tabulated. In general, the newer cephalosporins have an extended spectrum of activity against gram-negative bacteria, including Serratia marcescens, Pseudomonas aeruginosa, and Neisseria gonorrhea. They also tend to be active against anaerobes, including Bacteroides fragilis. However, they generally have less activity against gram-positive bacteria when compared with the first- and second-generation cephalosporins. Clinical summaries are given for each of the cephalosporins, with emphasis on the results of comparative clinical trials. These cephalosporins may prove especially useful in nosocomial infections with resistant organisms, intraabdominal infections, febrile episodes in the granulocytopenic patient, and meningitis.