The new acylampicillin derivatives azlocillin, mezlocillin, and piperacillin have an increased activity against many gram-negative bacilli, especially Klebsiella pneumoniae, Serratia marcescens, and Pseudomonas aeruginosa, when compared with the carboxypenicillins carbenicillin and ticarcillin. The new penicillins show synergistic activity in combination with aminoglycosides but, when combined with other beta-lactams, may be synergistic (piperacillin and moxalactam; mezlocillin and cefoperazone), indifferent, or antagonistic (azlocillin, mezlocillin, or piperacillin and cefoxitin or cefamandole). The in vitro activity of these agents, either alone or in combination, appears to correlate with in vivo efficacy in animal models. The new penicillins are clinically effective for a very broad range of infections, including life-threatening nosocomial infections. Adverse effects with these, as with other semisynthetic penicillins, are minimal. Attention must be paid to the potential for infection by naturally resistant, gram-negative bacilli such as beta-lactamase-producing Escherichia coli and for the emergence of resistance during therapy. The granulocytopenic patient should receive these agents only in conjunction with another agent, such as an aminoglycoside; this combination will often result in a synergistic effect when tested in vitro. The carboxypenicillins and the newer penicillins have substantial similarities, and prospective, comparative studies have so far failed to demonstrate significant clinical superiority. However, the increased activity of the acylampicillins may be advantageous for the treatment of infections due to K. pneumoniae and P. aeruginosa.