Agar dilution tests for susceptibility of gram-negative rods and enterococci were done with a number of penicillins, cephalosporins, lincomycin analogues, erythromycin, and rifampin. Many in the first three categories were investigational drugs. All were generally less active than aminoglycoside and tetracycline antibiotics against gram-negative rods and more active against enterococci. Cephalosporins as a group were more active than penicillins against Klebsiella pneumoniae and Escherichia coli and less active enterococci. Both groups were equally active against Enterobacter, Proteus, and Providencia but inactive against most strains of Serratia and all strains of Pseudomonas; however, ticarcillin, carbenicillin, and BL-1654 were active against most strains of Pseudomonas. Penicillins and cephalosporins were more active against Proteus mirabilis than against indole-positive Proteus. Lincomycins had little or no activity against gram-negative rods but were moderately active against enterococci. Erythromycin was more active than the lincomycins, but rifampin was much more active than either of these types of drug. Of the penicillins, ticarcillin, carbenicillin, and BL-P1654 were the most active against gram-negative rods, whereas BL-P1654, amoxicillin, and ampicillin were the most active against enterococci. The penicillinase-resistant penicillins, cyclacillin, and penicillin V were essentially inactive against gram-negative rods. Of the cephalosporins tested, cephanone and cefamandole were the most active against most gram-negative rods, whereas cephaloridine and cephacetrile were the most active against enterococci. The least active of the cephalosporins against most species were cephradine, cephalexin, and cephapirin, but cefoxitin was the least active against enterococci.