The origin of DNA in transducing particles of bacteriophage Mu was investigated by density labelling techniques. Unlabelled plaque-forming and leu+-transducing particles were of about the same density. Preinfection labelling of DNA with 5-bromodeoxyuridine increased the density of the transducing particles, but not that of the infective ones. Postinfection labelling increased the density of the infective particles twice as much as that of the transducing particles. We conclude that half of the transducing DNA is synthesized before infection and half is synthesized after infection, similar to the results obtained with P1kc transducing phages (Ikeda and Tomizawa, 1965).