1. A lysate of a high T3 titer was obtained by the following procedure. Escherichia coli B of logarithmic phase was centrifuged down and resuspended in a fresh medium at the concentration of ca. 5×109/ml and infected with T3 (multiplicity of infection =1/20) and shaken at 37°C. The phage titer of the resulting lysate reached usually 3-5×1011/ml.2. T3 can be concentrated from the lysate by ammonium sulfate precipitation or evaporation under reduced pressure.3. Highly purified T3 was recovered from DEAE-cellulose chromatography. A DEAE-cellulose column, washed thoroughly with a buffered saline (0.1M NaCl, 0.005M Tris pH 8), was charged with T3 suspended in the same solution. The column was washed with the same saline and T3 was then eluted with 0.2M NaCl, 0.005M Tris pH 8 in a good recovery.From the experiments of the isotope labeling, the phosphorous content of T3 was calculated to be 6×10-12γ P per plaque forming unit. The ultraviolet absorbancy of T3 was also determined. The optical density at 260mμ was 0.18 with the suspension of 1011/ml.4. The stability of the phage was investigated. T3 was less stable at low salt concentration and acidic pH (under pH 6).