Bacteriophages for Acinetobacter baumanii, Pseudomonas aeruginosa and Staphylococcus aureus were tested in experimental infections of mice to investigate their potential for the treatment of infections of man. As few as 10(2) particles of an acinetobacter phage protected mice against 5 LD50 (1 x 10(8)) of a virulent strain of A. baumanii, and phage was demonstrated to have multiplied in the mice. A pseudomonas phage protected mice against 5 LD50 of a virulent strain of P. aeruginosa, with a PD50 of 1.2 x 10(7) particles. A staphylococcal phage failed to protect mice infected with a strain of S. aureus. These studies support the view that bacteriophages could be useful in the treatment of human infections caused by antibiotic-resistant strains of bacteria.