A pure preparation of the staphylococcal typing bacteriophage-42-D.

The staphylococcal typing-phage 42 D is propagated on bacteria which are lysogenic for the phage 1363, and the latter phage is present as a contaminant in preparations of phage 42D. This contamination can be avoided by propagating a pure clone of phage 42D on non-lysogenic organisms. Such nonlysogenic propagating strains can be isolated after the lysogenic propagating organisms have been irradiated heavily with ultraviolet radiation.