Recently, a newly designed encapsulated source of 125I has become commercially available for use in permanent and temporary interstitial brachytherapy. The 125I sources in current use come in two different configurations: the Model 6711 source (Medi Physics/Amersham) for permanent implants has radioactive iodine adsorbed on the surface of a silver wire, and the Model 6702 (Medi Physics/Amersham) source for temporary implants has radioactive iodine absorbed in three spherical resin balls. Both of these iodine sources are encapsulated in a thin-walled shell (0.05-mm thick) made of titanium. The newly designed 125I source (Best Industries Model 2300 series) contains radioactive iodine adsorbed on a tungsten wire that is encapsulated by two walls of titanium. This double-walled 125I source offers the following potential advantages: (i) Because it contains radioactive iodine on the ends as well as the circular surface of the tungsten wire, it can produce a more isotropic dose distribution than the sources in current use; (ii) because it is available in a wider range of source strengths, it is suitable for both temporary and permanent implantation; (iii) because it has a tungsten radiographic marker, source localization is considerably easier than the 125I Model 6702 source that has no radiographic marker; and (iv) because it uses a double-walled encapsulation the risk of radioactive contamination due to source rupture is considerably reduced. In this work, dose distributions produced by the new design 125I source (Model 2300) for interstitial brachytherapy have been measured using LiF TLD's in a Solid Water phantom. Dosimetric characteristics of the new 125I sources are compared with those of the currently available 125I sources.(ABSTRACT TRUNCATED AT 250 WORDS)