Of all patients hospitalized at Bronx-Lebanon Hospital Center during a six-month period, 7.9% had infections and 3.6% had nosocomial infections. Three fourths of all organisms from nosocomial infections were gram-negative and Escherichia coli, Klebsiella and Proteus represented three fourths of all gram-negative organisms. A larger variety of species was observed in infections following antibiotic treatment plus instrumentation than in infections preceded by only one or the other factor. Antibiotic sensitivity tests on all organisms revealed the highest percentage of resistance to ampicillin and tetracycline and the greatest degree of sensitivity to gentamicin, chloramphenicol, kanamycin sulfate, and cephalothin. Comparing the isolates from nosocomial infections to those from community-acquired infections, the gram-negative organisms from the former were more resistant to antibiotics than those from the latter, and gram-positive organisms were equally resistant from both places.