The potential synergistic effect of piperacillin with gentamicin was examined against five isolates each of Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus faecalis isolated from patients with endocarditis. Piperacillin and gentamicin synergistically inhibited all strains of P. aeruginosa and S. aureus tested. They exerted no synergistic effect against S. faecalis. Piperacillin and gentamicin synergism may be clinically tested in the treatment of serious pseudomonas and staphylococcal infections but not against infections caused by S. faecalis.