Bacteriophages (viruses of bacteria) are currently considered a promising means of treating antibiotic-resistant infections. The main objective of this study was to evaluate the intensity of the whole-blood monocyte and neutrophil respiratory burst induced by purified preparations and lysates of the bacteriophages T4 and A3/R. While A3/R phage preparations did not induce a significant respiratory burst, T4 phage preparations increased the production of reactive oxygen species in a dose-dependent manner. However, the intensity of the phage-induced respiratory burst was much lower than that triggered by heat-inactivated Staphylococcus aureus cells. These results suggest that phage preparations are not likely to induce oxidative stress following their administration to patients.